Get tests running on Python 3

This pull requests gets the unit tests to pass while running under Python 3. The code had a bunch of version checks scattered throughout to maintain compatibility -- I took those out and used the [`six`](http://pythonhosted.org/six/) library instead, which is designed for that. I also had to modify some of the number formatting in a few finicky tests; the formatting seemed pretty arbitrary, so I didn't think there would be any issue with me modifying it.

Once this is merged, can you enable Travis and Coveralls support by signing in with your "embolalia" Github account on their respective websites, and switching on the `willie` repo for each?