Import/export modulestore-stored asset metadata as XML.

Export modulestore-stored asset metadata as XML to exported course.
Import asset metadata XML from exported course into modulestore.
Minor changes to asset XML schema definition.
Rather than list all the asset types that should be exported here, can you have the client of the asset-store specify whether their asset type should be exported?  Maybe `exportable` is a meta data stored with each entry?  Or maybe different asset types are registered with this service?