Remove Redis dependencies from dev and improve requirements

This removes the dependencies on Redis & celery from settings/base.py and puts them to settings/production.py. It also moves django-debug-toolbar INSTALLED_APP to settings/dev.py and djcelery and kombu to settings/production.py.

This also improves the way requirements.txt is handled: Instead of a single file a requirements folder is created containing base.txt, dev.txt and production.txt each one containing the appropriate requirements.

One question: Is fabric *really* required ? I don't see a ``fabfile.py``.