This allows XModuleDescriptors to export themselves to XML

These changes make the LMS read from the XML course files using the ModuleStore interface. 

Note that stable-edx4edx will now no longer be a clean merge.  But this code has priority.

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)