Add support for using tox to run tests

Summary:
tox aims to automate and standardize testing in Python.  See docs:
http://tox.readthedocs.org/

If you have tox installed (`pip install tox`), you can now run the tests
simply by running `tox` in the same directory as tox.ini.

Test Plan: Installed tox; Ran tox.

Reviewers: emfree, kav-ya

Reviewed By: kav-ya

Subscribers: khamidou, siro

Differential Revision: https://review.inboxapp.com/D657