Add tox config

I've added a tox config so that it's possible to run the test locally. I kept compatibility with older versions of Django but I wanted to get your feedback on backwards compatibility for other changes I make.

Part of my motivation for this pull request is that I want to get django_polymorphic working with Django 1.5+. Can you tell me which versions of Django I should be keeping support for when I make pull requests? Personally, I think it makes sense to only support currently maintained versions of Django (and maybe the development branch when things are close to release). Right now, that would translate to Django 1.4, 1.5 and may dev (what will be 1.6) in a branch. What do you think?

I can rework the parts of this pull request to remove the bits that keep Django < 1.4 compatibility. At least we should consider removing support for Django < 1.3. Let me know.

@vdboor Any thoughts on this?

Thanks, Ben