Make split mongo read-only API consistent with other modulestores.
Make it clear that get_course_for_item is only a helper method in mongo modulestore.
Add get_instance method.
pylint/pep8 cleanup.

cleanup.
Make get_errored_courses part of the modulestore API (with default implementation).

Also clarifies the contraction of location.course_id by throwing an exception for lcoations that are not of category course.

Add test for course_id method.
Merge pull request #667 from edx/christina/read-only-api

Make split mongo read-only API consistent with other modulestores.