Add tox, pytest, support for pypy, py33, py34, py26.  Update setup.py

This PR should be complete with support for the specified platforms
I believe this should use a ``>=`` instead of a ``==``, to avoid downgrading needlessly.

Also, the version number should probably be the minimal one that works (it doesn't need to match the one in requirements.txt), to avoid upgrading needlessly -- keeping in mind that installing/upgrading modules can be weird for some users, specially on Windows.

Does that make sense?