Add Travis and Tox

You'll need to create a (free) Travis account on https://travis-ci.org, and then activate Travis on your repository.

This only tests python 2.6 and 2.7, I'll try to create another PR for python 3.3 and python 3.4 (tests aren't passing at the moment).

Also, when developing locally, you can now simply run `tox` (after a `pip install tox`) to automatically run the tests on all the environments.