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
We need to figure out how to consolidate our docstrings for each modulestore class, so they are either defined just in one place, or kept in sync everywhere.  In this file, some overridden methods have docstrings and some don't.