Python 3

* Set up tox
* Support Django custom user model
* Updates to support versions Python 2.6, 2.7 and 3.3
* Flake8 fixes (one of which was a real, though inconsequential, bug)

I took the liberty of including the version bump in the PR. If approved, I'll tag and see if I can figure out how to push it to PyPI.

Thanks to @ironfroggy and @dpoirier for the hard parts.