Use a string key for default groups in abtests (and remove code that was expecting groups from django)

Conflicts:
	lms/djangoapps/courseware/grades.py

Make location into a named tuple, and use it more as a first class entity, rather than URL for identifying content

These changes make xml import into the cms use XModuleDescriptors

Course settings is in json, not python

Make user profiles work again after the switch to an XMLModuleStore. Staff user histograms are still broken