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.
The api doc isn't clear, but it looks like for on an lxml element iterates over all nodes in the subtree. Perhaps change to `asset_xml.iterchildren()` just to be clear? I'm surprised there's no `asset_xml[AssetMetadata.ASSET_TYPE_ATTR]` to get the specific child directly.