Run tests on Travis CI

This patch is supposed to run tests using the Travis CI, on several releases of Django and Python. As a bonus, tests are automatically executed for each pull request so that you don't end up accidentily merging into a known broken state.

Please switch on Travis *before* merging this one so we see whether it works - sometimes getting Travis to behave well takes some tweaking (I can't because I'm a mere contributor and not the repo owner ^^).

https://travis-ci.org/