Tox tests

Hi,

These changes add tox as a test dependency and configure Travis to use tox to run the tests against Django 1.5, 1.6 and 1.7.

For Django 1.7 I had to tweak the runtests.py to setup things properly. For Django 1.5 I removed some assertNumQueries assertions which were failing, even though the logic for hashbrown itself was correct.

I was thinking that you could just use Travis to configure the tests against different Django versions, but the nice thing about using tox is it is a heck of a lot simpler to test locally.

David B.