Python 3 compatibility + basic tests

Hi Dan,
I've attempted to make `kplr` Python 3 compatible. I introduced a dependency on the `six` library and added some pretty straightforward test cases passing under both 2.7 and 3.3 (tox config included). More extensive test coverage will require mocking EA and MAST responses and I'm not sure if this is the way to go. Anyway, I had no problems running the examples from the documentation under Python 3.3 after these changes.