Fix for Celery >= 3.0 with 2.x fallback and Tox testing for 2.x and 3.x

Fix for the celery 3.x issue with Tox tests. Restructured setup.py a bit for Tox testing. You can now run tox tests by running python setup.py test. Tox ini has cases for celery 2.x and 3.x. 

I did not get the RabbitMQ tests running because I wasn't sure how they needed to be configured, so for me those 2 tests fail. But I'm assuming they will be fine with the proper setup.