Add support for tox (http://tox.testrun.org/)

Sample tox output:

```
[last: 3] marca@scml-marca:~/dev/git-repos/slumber$ tox
GLOB sdist-make: /Users/marca/dev/git-repos/slumber/setup.py
py27 sdist-reinst: /Users/marca/dev/git-repos/slumber/.tox/dist/slumber-0.4.2.zip
py27 runtests: commands[0]
Requirement already satisfied (use --upgrade to upgrade): requests in ./.tox/py27/lib/python2.7/site-packages (from -r requirements.txt (line 1))
Requirement already satisfied (use --upgrade to upgrade): simplejson in ./.tox/py27/lib/python2.7/site-packages (from -r requirements.txt (line 2))
Cleaning up...
py27 runtests: commands[1]
Requirement already satisfied (use --upgrade to upgrade): mock in ./.tox/py27/lib/python2.7/site-packages (from -r requirements-test.txt (line 1))
Cleaning up...
py27 runtests: commands[2]
running test
running egg_info
writing requirements to slumber.egg-info/requires.txt
writing slumber.egg-info/PKG-INFO
writing top-level names to slumber.egg-info/top_level.txt
writing dependency_links to slumber.egg-info/dependency_links.txt
reading manifest file 'slumber.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
writing manifest file 'slumber.egg-info/SOURCES.txt'
running build_ext
test_get_serializer (resource.ResourceTestCase) ... ok
test_request_200 (resource.ResourceTestCase) ... ok
test_url_join_decoded_unicode (utils.UtilsTestCase) ... ok
test_url_join_encoded_unicode (utils.UtilsTestCase) ... ok
test_url_join_http (utils.UtilsTestCase) ... ok
test_url_join_http_port (utils.UtilsTestCase) ... ok
test_url_join_https (utils.UtilsTestCase) ... ok
test_url_join_https_port (utils.UtilsTestCase) ... ok
test_url_join_path (utils.UtilsTestCase) ... ok
test_url_join_trailing_slash (utils.UtilsTestCase) ... ok

----------------------------------------------------------------------
Ran 10 tests in 0.002s

OK
pypy sdist-reinst: /Users/marca/dev/git-repos/slumber/.tox/dist/slumber-0.4.2.zip
pypy runtests: commands[0]
Requirement already satisfied (use --upgrade to upgrade): requests in ./.tox/pypy/site-packages (from -r requirements.txt (line 1))
Requirement already satisfied (use --upgrade to upgrade): simplejson in ./.tox/pypy/site-packages (from -r requirements.txt (line 2))
Cleaning up...
pypy runtests: commands[1]
Requirement already satisfied (use --upgrade to upgrade): mock in ./.tox/pypy/site-packages (from -r requirements-test.txt (line 1))
Cleaning up...
pypy runtests: commands[2]
running test
running egg_info
writing requirements to slumber.egg-info/requires.txt
writing slumber.egg-info/PKG-INFO
writing top-level names to slumber.egg-info/top_level.txt
writing dependency_links to slumber.egg-info/dependency_links.txt
reading manifest file 'slumber.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
writing manifest file 'slumber.egg-info/SOURCES.txt'
running build_ext
test_get_serializer (resource.ResourceTestCase) ... ok
test_request_200 (resource.ResourceTestCase) ... ok
test_url_join_decoded_unicode (utils.UtilsTestCase) ... ok
test_url_join_encoded_unicode (utils.UtilsTestCase) ... ok
test_url_join_http (utils.UtilsTestCase) ... ok
test_url_join_http_port (utils.UtilsTestCase) ... ok
test_url_join_https (utils.UtilsTestCase) ... ok
test_url_join_https_port (utils.UtilsTestCase) ... ok
test_url_join_path (utils.UtilsTestCase) ... ok
test_url_join_trailing_slash (utils.UtilsTestCase) ... ok

----------------------------------------------------------------------
Ran 10 tests in 0.004s

OK
_________________________________________________________________________ summary __________________________________________________________________________
  py27: commands succeeded
  pypy: commands succeeded
  congratulations :)
```