Fix test failures under Python 2.6

As mentioned in #26 this pull request fixes issues which occurred under Python 2.6:

1. Relying on unittest functionality which is only available in Python >= 2.7 (I've added `unittest2` dependency which gets installed when the tests are ran under 2.6)
3. Relying on set literals which only available in Python >= 2.7

Test runner output before this change: https://gist.github.com/Kami/6530053
Test runner output after this change: https://gist.github.com/Kami/6530059

Note: This pull request is based off `add_tox_and_travis_ci_config` branch (#26).