Support Python 2 and Python 3 with single source

Right now you have to install `unittest2` for Python 2 and `unittest2py3k` for Python 3.

This is a little clunky when using pip requirements files and tox because you have to have two separate requirements files and two separate tox targets.

It would be nice if there were one package that supported both.

Google Code issue: https://code.google.com/p/unittest-ext/issues/detail?id=81
Bitbucket Mercurial branch: https://bitbucket.org/msabramo/unittest2/branch/py2_and_py3_single_source