Tox config

Tests against Python 2.6-2.7, and 3.3.
Django 1.4-1.6

If it's ok, I can update [travis.yml to use tox too](http://alexgaynor.net/2014/jan/06/why-travis-ci-is-great-for-the-python-community/).

I've been using tox to develop python 2&3 simultaneous support (#96)