Import/export modulestore-stored asset metadata as XML from/to exported course.

Export modulestore-stored asset metadata as XML to exported course.
Import asset metadata XML into course's modulestore.
Return empty values and log a warning instead of raising NotImplemented exceptions for XML modulestore asset metadata methods.
Optimize importing many items of asset metadata by avoiding multiple round-trips to MongoDB.
in particular: `etree.parse` (https://docs.python.org/2/library/xml.etree.elementtree.html#xml.etree.ElementTree.parse)