Integrate test suite with tox

This should make running the test suite much easier :) I've moved the settings out of the schemamigration.py script and into a test_settings.py module that can then be used by the script and the test-runner.
That won't work since the script is not inside a package :) Or do you always add the whole repo to your PYTHONPATH before executing this script? (To be honest, I'm not quite certain about the purpose of this script :))