Acknowledge the fact that right now keystore is really just a module store. If we need a keystore that returns other objects, we can reexctract the base class into it's own module again

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