Add tox test runner

This is my first experience with Tox.  It seems to me that you have to move the tests into the package itself so that tox can find them.  Hence I've moved ``tests.py`` into the package as well as some test settings.  I'm not sure if this is desirable - I've always preferred to keep the tests separate where possible.

The ``tox.ini`` will probably need extending with all the required combinations of python and django.