tox.ini: use py.test directly instead of python setup.py test

This is a similar change to #292, but based on the branch in #293 (brings over `tox.ini` changes from @sigmavirus24 on the `1.0-alpha` branch).

```
$ tox -e py27
GLOB sdist-make: /Users/marca/dev/git-repos/github3.py/setup.py
py27 inst-nodeps: /Users/marca/dev/git-repos/github3.py/.tox/dist/github3.py-0.9.2.zip
py27 runtests: PYTHONHASHSEED='2339110401'
py27 runtests: commands[0] | py.test -q tests/
..............................................x...............................................................................................................................................................................................................................................................................................................................................................................................................................................................
493 passed, 1 xfailed in 3.79 seconds
___________________________________________________________________________________ summary ____________________________________________________________________________________
  py27: commands succeeded
  congratulations :)

$ tox -e py27 -- --tb=short --pdb --pyargs tests.integration.test_repos_release -k test_download
GLOB sdist-make: /Users/marca/dev/git-repos/github3.py/setup.py
py27 inst-nodeps: /Users/marca/dev/git-repos/github3.py/.tox/dist/github3.py-0.9.2.zip
py27 runtests: PYTHONHASHSEED='3359558432'
py27 runtests: commands[0] | py.test --tb=short --pdb --pyargs tests.integration.test_repos_release -k test_download
============================================================================= test session starts ==============================================================================
platform darwin -- Python 2.7.6 -- py-1.4.26 -- pytest-2.6.4
collected 6 items

tests/integration/test_repos_release.py .

=================================================================== 5 tests deselected by '-ktest_download' ====================================================================
==================================================================== 1 passed, 5 deselected in 0.09 seconds ====================================================================
___________________________________________________________________________________ summary ____________________________________________________________________________________
  py27: commands succeeded
  congratulations :)
```
The reason that I moved the defaults from the `pytest` section to a `testenv` section might be subtle and not immediately obvious, so I'll explain just in case. 

With a `pytest` section, AFAIK, that is always applied and I know of no way to override it. Where this is really problematic is when you put something like `tests/` in there. If you have that and then use the posargs stuff to tell pytest to run a subset of tests, it is additive instead of replacing. IOW, it will run all of the tests in `tests/` and then the ones on the command-line. On the contrary, {posargs:-q tests/} will default to `-q tests/` but REPLACE that if options are supplied on the tox command-line after a double-hyphen.