TST: Add support for TOX

[parthea]

It could be helpful to setup tox for this repo. Once it is setup simply running tox at the root of the repo will run tests in both versions, as well as flake8.

[tswast]

I added this in #160. I added some light instructions for using it at https://github.com/pydata/pandas-gbq/blob/master/docs/source/contributing.rst#testing-on-multiple-python-versions