Testing with tox (and coverage)

Travis is nice, but as a hosted solution it requires to commit and push the code. Personally I prefer to test things before committing.

This changeset allows to test with [tox](http://tox.testrun.org/latest/index.html) against 3 different Django versions (1.3.x, 1.4.x and development). I've also fixed packaging a bit — prevented `example` from installing in `site-packages` (but obviously kept it in sdist) and fixed a typo in `MANIFEST.in` that prevented fixtures from installing.