Run tests on Python 3.3/2.7/PyPy with Tox, run tests on Travis

This just takes your existing tests and runs them:
 + through `setup.py test`
 + through `tox`. By default, the tox runs don't assume that mongodb is installed, but they do install sqlalchemy.
 + through Travis CI (if you enable it) - this tests mongodb as well, because Travis supports it.

It depends on my previous Python 3 pull request, sorry. If that's a problem I can disaggregate them.