Python 3 support

Adds support for Python 3, specifically Python 3.3.

* No support for Python 3.0, 3.1, or 3.2 -- it's not worth the added effort. Anyone using Py3 is on the cutting edge anyway, so getting them to use 3.3 is fine.

* Drops support for Django 1.3 because Django 1.3 doesn't have `django.utils.six` available. If 1.3 support is important, I can make that happen, let me know.

* Drops support for Python 2.5. I'm not sure 2.5 and 3.3 support is possible in a single source tree, but again if it's important I'll give it a shot.

* I'm not convinced I got the changes to the Travis config right; might need some tweakage.