Add tests for usage with Flask

[winhamwr]

The test suite currently only runs with Django in existence, which is bad because we won't detect breakages for Flask or other non-Django Celery uses.

Django-Celery's setup.py is probably a good inspiration for how to fix this.

1. Add a custom command (test_flask?) that runs some tests based on Flask project.
2. If we need to create a tiny Flask project to get the tests running, that can live at test_projects/flask or some such.
3. Then we can use a tox environment with a requirements/flask_test.txt file to ensure things are kosher without Django even being installed.