Add support for testing with tox

Excellent work on porting bleach to work on Python 3! Thanks!

Here's a minor addition that might help. It adds support for a testing tool called [tox](http://testrun.org/tox/latest/).

Tox lets you test across multiple Python versions quickly and easily (and without having to commit and push to GitHub like with Travis CI [so IMHO having both is nice]).

I'm also submitting this as https://github.com/jsocol/bleach/pull/103, so you might at some point get the same changes if you merge from jsocol and he has merged that PR.

Thanks again for making this port!