Use tox to run tests on multiple Python VMs

You can run tests on all to-be-supported Python VMs using [tox][] or [detox][].

![screen shot 2014-11-05 at 2 39 58 pm](https://cloud.githubusercontent.com/assets/12431/4913279/33d18dba-64ae-11e4-8930-cf4387fe8ede.png)

[tox]: https://testrun.org/tox/
[detox]: https://pypi.python.org/pypi/detox