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
I went around and around on this thinking there had to be a better way. If I do what you say, `fields` will end up being `[{field1: value, field2: value2}, {field3: value3, field4: value4}, {field5: value5}]` instead of a single dict