Add tox.ini.

Currently this targets Django 1.4 & 1.5 on Python 2.6 & 2.7 as we'll
drop support for Django 1.3 and Python 2.5 in the next release.

Python 3.2 & 3.3 isn't supported yet.
Merge pull request #409 from aaugustin/add-tox-file

Add tox.ini. Fix #407.