Add python 2 and python 3 testing through tox

[aronasorman]

This module is designed to run on both major versions of python! But unfortunately I haven't been able to write the tox config that would enable that.

@benjaoming any pointers? Feel free to add this too :)

[benjaoming]

Went ahead with this in #8, actually didn't see this issue :) But can be closed now :)