Phase 1 of adding asset metadata saving to old Mongo

@dmitchell @cpennington Here's the first phase of my work. Look at the tests to see what is implemented and what isn't yet.
@cpennington that's ambiguous, I think you meant, `unicode(course_key)`? or did you just mean `SON([('org', org), ('course', course), ('run', run)])`? Regardless, I'm glad he's not using this as the primary key in the new collection! I think he's using the `_id` as a primary key join column, but that's a carry over from sql which probably isn't necessary.