Add a 'MixedModuleStore' which aggregates between XML and Mongo Modulestores

This is an early PR while I work on manual/automated testing.

This should allow us to do away with:

1) Stubs of any kind
2) Running separate LMS-Mongo and LMS-XML services
3) Nginx rules to route requests

@dmitchell @cpennington @jarv @e0d Can you all start to take a look? I'm still manually validating right now and I need to figure out some unit tests around this. But I figured I'd get the review process started.

Also, we need to come up with production configuration settings. Right now dev configuration is in lms/envs/mixed_dev.py so to run locally `rake lms[cms.mixed_dev]`