Mattdrayer/api modulesgroups

It is now possible to link groups with courseware modules via the included API List/Detail operations.  A new model has been introduced to maintain the relationships:  ModuleGroupRelationship.  Rather than creating a new "projects" URI I've opted to expose the generic entities for now -- we can always subsume the orchestration and requisite network calls in a follow-on PR.  In addition we were going back and forth on what to call the model, and I decided to stick with the pre-existing modules concept which allowed me to work with the URI scheme that is already in place.  @chrisndodge @ormsbee @ziafazal, take a look and let me know what you think.