Use tox for testing

Tox is basically your local travis. It lets developers run tests for a whole build matrix in a single command.

Changes:

* Add Travis build status image (a bit of bragging doesn't hurt)
* Integrate tox for running the tests locally and on travis
* Add a build target for docs with sphinx's `-W` option. Doc patches which trigger sphinx warnings will break travis :)
* Add a section in the docs about setting up a local dev environment, running/writing tests, etc.
* Fix some Django warnings

One part I'm not 100% happy about: for tox to work one needs to remove `setup.cfg` locally (or comment the `[install]` section). On Travis it's no big deal (done in `travis.yml`), locally this adds more risk to accidentally commit the file removal. Despite this small annoyance I think it's a nice improvement. If all changes go through pull requests a removal will be easy to spot.