Add tox.ini

This tests python 2.6 and 2.7 with Django 1.4 and 1.5.

The tests all pass fine with django 1.4, but they fail with django
1.5.

NB: I'm "faking" the python 2.6 tests on my system since I don't
have python 2.6 available. But travis is running 2.6 and 2.7, so
there you go.