Setup travis and tox.

To use travis, create an account on https://travis-ci.org/ and enable it on your repository. It'll automatically add a web hook to github on this repository, and run tox for all the supported versions on each commit.

If you don't need/want travis, also remove the "travis build status image" in the README.rst.

Tox is really convenient to make sure the tests are running correctly on all supported versions of python and django.