Addressed naming review notes in LibraryRoot XBlock
Refactored split modulestore to use helper methods instead of duplicated code in get_courses/get_libraries and get_course/get_library
* Fixed docstring for get_libraries
* Added contract definition for get_library
* Refactored get_library to use `_verify_modulestore_support`
Documented mongo calls when creating library + little nitpick in factories.py