Replace create_xmodule with create_xblock

@cpennington @nasthagiri please review

* The primary task of this is to do all of the import/export; so, the title's wrong
* reconciles the signature of create_xblock
* uses make_course_key from cale's branch
* uses nimisha's asset fixes
* lots of import/export fixes
Yes, originally, split had fields as dict to serialized values but when I added it to old mongo I made it a dict to pythonic values and deferred serialization to the modulestore.