Issue #89: Use Tox

First pull request, apologies if I've done something incorrectly.  Appreciate all feedback/criticisms, etc. 

```shell

(venv)➜  github3.py git:(develop) ✗ tox
GLOB sdist-make: /home/bmorriso/localrepository/github3.py/setup.py
py26 inst-nodeps: /home/bmorriso/localrepository/github3.py/.tox/dist/github3.py-0.6.1.zip
py26 runtests: commands[0]
...................................................................................................................................................................................................................................................................................................................................................................................
----------------------------------------------------------------------
Ran 371 tests in 4.123s

OK
Name                       Stmts   Miss  Cover
----------------------------------------------
github3/__init__               9      0   100%
github3/api                   69      0   100%
github3/auths                 47      0   100%
github3/decorators            30      0   100%
github3/events                92      0   100%
github3/gists/__init__         2      0   100%
github3/gists/comment          7      0   100%
github3/gists/file            10      0   100%
github3/gists/gist            82      0   100%
github3/gists/history         21      0   100%
github3/git                   80      0   100%
github3/github               386      0   100%
github3/issues/__init__       18      0   100%
github3/issues/comment         8      0   100%
github3/issues/event          18      0   100%
github3/issues/issue         104      0   100%
github3/issues/label          29      0   100%
github3/issues/milestone      39      0   100%
github3/legacy                64      0   100%
github3/models               203      0   100%
github3/notifications         59      0   100%
github3/orgs                 142      0   100%
github3/pulls                132      0   100%
github3/repos/__init__         2      0   100%
github3/repos/branch          10      0   100%
github3/repos/comment         28      0   100%
github3/repos/commit          30      0   100%
github3/repos/comparison      27      0   100%
github3/repos/contents        27      0   100%
github3/repos/download        31      0   100%
github3/repos/hook            46      0   100%
github3/repos/repo           494      0   100%
github3/repos/status          14      0   100%
github3/repos/tag             10      0   100%
github3/structs               48      0   100%
github3/users                121      0   100%
----------------------------------------------
TOTAL                       2539      0   100%
py27 inst-nodeps: /home/bmorriso/localrepository/github3.py/.tox/dist/github3.py-0.6.1.zip
py27 runtests: commands[0]
...................................................................................................................................................................................................................................................................................................................................................................................
----------------------------------------------------------------------
Ran 371 tests in 0.963s

OK
Name                       Stmts   Miss  Cover
----------------------------------------------
github3/__init__               9      0   100%
github3/api                   69      0   100%
github3/auths                 47      0   100%
github3/decorators            30      0   100%
github3/events                92      0   100%
github3/gists/__init__         2      0   100%
github3/gists/comment          7      0   100%
github3/gists/file            10      0   100%
github3/gists/gist            82      0   100%
github3/gists/history         21      0   100%
github3/git                   80      0   100%
github3/github               386      0   100%
github3/issues/__init__       18      0   100%
github3/issues/comment         8      0   100%
github3/issues/event          18      0   100%
github3/issues/issue         104      0   100%
github3/issues/label          29      0   100%
github3/issues/milestone      39      0   100%
github3/legacy                64      0   100%
github3/models               203      0   100%
github3/notifications         59      0   100%
github3/orgs                 142      0   100%
github3/pulls                132      0   100%
github3/repos/__init__         2      0   100%
github3/repos/branch          10      0   100%
github3/repos/comment         28      0   100%
github3/repos/commit          30      0   100%
github3/repos/comparison      27      0   100%
github3/repos/contents        27      0   100%
github3/repos/download        31      0   100%
github3/repos/hook            46      0   100%
github3/repos/repo           494      0   100%
github3/repos/status          14      0   100%
github3/repos/tag             10      0   100%
github3/structs               48      0   100%
github3/users                121      0   100%
----------------------------------------------
TOTAL                       2539      0   100%
py31 inst-nodeps: /home/bmorriso/localrepository/github3.py/.tox/dist/github3.py-0.6.1.zip
py31 runtests: commands[0]
...............s...................................................................................................................................................................................................................................................................................................................................................................
----------------------------------------------------------------------
Ran 371 tests in 3.889s

OK (skipped=1)
Name                       Stmts   Miss  Cover
----------------------------------------------
github3/__init__               9      0   100%
github3/api                   69      0   100%
github3/auths                 47      0   100%
github3/decorators            30      0   100%
github3/events                92      0   100%
github3/gists/__init__         2      0   100%
github3/gists/comment          7      0   100%
github3/gists/file            10      0   100%
github3/gists/gist            82      0   100%
github3/gists/history         21      0   100%
github3/git                   80      0   100%
github3/github               386      0   100%
github3/issues/__init__       18      0   100%
github3/issues/comment         8      0   100%
github3/issues/event          18      0   100%
github3/issues/issue         104      0   100%
github3/issues/label          29      0   100%
github3/issues/milestone      39      0   100%
github3/legacy                64      0   100%
github3/models               203      0   100%
github3/notifications         59      0   100%
github3/orgs                 142      0   100%
github3/pulls                132      0   100%
github3/repos/__init__         2      0   100%
github3/repos/branch          10      0   100%
github3/repos/comment         28      0   100%
github3/repos/commit          30      0   100%
github3/repos/comparison      27      0   100%
github3/repos/contents        27      1    96%
github3/repos/download        31      0   100%
github3/repos/hook            46      0   100%
github3/repos/repo           494      0   100%
github3/repos/status          14      0   100%
github3/repos/tag             10      0   100%
github3/structs               48      0   100%
github3/users                121      0   100%
----------------------------------------------
TOTAL                       2539      1    99%
py32 inst-nodeps: /home/bmorriso/localrepository/github3.py/.tox/dist/github3.py-0.6.1.zip
py32 runtests: commands[0]
...............s...................................................................................................................................................................................................................................................................................................................................................................
----------------------------------------------------------------------
Ran 371 tests in 3.836s

OK (skipped=1)
Name                       Stmts   Miss  Cover
----------------------------------------------
github3/__init__               9      0   100%
github3/api                   69      0   100%
github3/auths                 47      0   100%
github3/decorators            30      0   100%
github3/events                92      0   100%
github3/gists/__init__         2      0   100%
github3/gists/comment          7      0   100%
github3/gists/file            10      0   100%
github3/gists/gist            82      0   100%
github3/gists/history         21      0   100%
github3/git                   80      0   100%
github3/github               386      0   100%
github3/issues/__init__       18      0   100%
github3/issues/comment         8      0   100%
github3/issues/event          18      0   100%
github3/issues/issue         104      0   100%
github3/issues/label          29      0   100%
github3/issues/milestone      39      0   100%
github3/legacy                64      0   100%
github3/models               203      0   100%
github3/notifications         59      0   100%
github3/orgs                 142      0   100%
github3/pulls                132      0   100%
github3/repos/__init__         2      0   100%
github3/repos/branch          10      0   100%
github3/repos/comment         28      0   100%
github3/repos/commit          30      0   100%
github3/repos/comparison      27      0   100%
github3/repos/contents        27      1    96%
github3/repos/download        31      0   100%
github3/repos/hook            46      0   100%
github3/repos/repo           494      0   100%
github3/repos/status          14      0   100%
github3/repos/tag             10      0   100%
github3/structs               48      0   100%
github3/users                121      0   100%
----------------------------------------------
TOTAL                       2539      1    99%
py33 inst-nodeps: /home/bmorriso/localrepository/github3.py/.tox/dist/github3.py-0.6.1.zip
py33 runtests: commands[0]
...............s...................................................................................................................................................................................................................................................................................................................................................................
----------------------------------------------------------------------
Ran 371 tests in 4.342s

OK (skipped=1)
Name                       Stmts   Miss  Cover
----------------------------------------------
github3/__init__               9      0   100%
github3/api                   69      0   100%
github3/auths                 47      0   100%
github3/decorators            30      0   100%
github3/events                92      0   100%
github3/gists/__init__         2      0   100%
github3/gists/comment          7      0   100%
github3/gists/file            10      0   100%
github3/gists/gist            82      0   100%
github3/gists/history         21      0   100%
github3/git                   80      0   100%
github3/github               386      0   100%
github3/issues/__init__       18      0   100%
github3/issues/comment         8      0   100%
github3/issues/event          18      0   100%
github3/issues/issue         104      0   100%
github3/issues/label          29      0   100%
github3/issues/milestone      39      0   100%
github3/legacy                64      0   100%
github3/models               203      0   100%
github3/notifications         59      0   100%
github3/orgs                 142      0   100%
github3/pulls                132      0   100%
github3/repos/__init__         2      0   100%
github3/repos/branch          10      0   100%
github3/repos/comment         28      0   100%
github3/repos/commit          30      0   100%
github3/repos/comparison      27      0   100%
github3/repos/contents        27      1    96%
github3/repos/download        31      0   100%
github3/repos/hook            46      0   100%
github3/repos/repo           494      0   100%
github3/repos/status          14      0   100%
github3/repos/tag             10      0   100%
github3/structs               48      0   100%
github3/users                121      0   100%
----------------------------------------------
TOTAL                       2539      1    99%
______________________________________________________________________ summary ______________________________________________________________________
  py26: commands succeeded
  py27: commands succeeded
  py31: commands succeeded
  py32: commands succeeded
  py33: commands succeeded
  congratulations :)
```