Add support for Tox (http://tox.testrun.org/) and Travis CI (http://about.travis-ci.org/)

This adds the ability to run the tests against multiple versions of Python using [Tox](http://tox.testrun.org/) (a local tool) and [Travis CI](http://about.travis-ci.org/) (a cloud-based CI system).

This was done with an eye towards perhaps eventually making `hexagonit.recipe.cmmi` work with Python 3 -- not sure if folks are already working on that?