Add supported language version classifiers

[Hwesta]

Updated the setup.py to correctly list the Python 2/3 support in the classifiers. Currently, it shows up as not supporting Python 3 on PyPI and related tools since no language versions are listed. Information from https://travis-ci.org/samgiles/slumber

[coveralls]

Coverage remained the same at 87.215% when pulling 78612c2 on Hwesta:add-language-classifiers into 4c4b7bc on samgiles:master.

[coveralls]

Coverage remained the same at 87.215% when pulling 78612c2 on Hwesta:add-language-classifiers into 4c4b7bc on samgiles:master.

[coveralls]

Coverage remained the same at 87.215% when pulling 78612c2 on Hwesta:add-language-classifiers into 4c4b7bc on samgiles:master.

[Hwesta]

Despite the tox.ini saying to explicitly install coverage==3.7.1, the current version (4.2) that doesn't support python 3.2 is being installed and erroneously failing the tests.