Support for exporting with IMS Content Packing

QTI 2.0 and Desire2Learn formats use IMS Content Packaging to include files with their question definitions. For example, an image referred to in the question or a movie to watch first.

When we export to QTI 2.0, per the spec, we should be outputting a CP zip to wrap the XML. It should also include a content manifest. And ideally, it would include the content files as well.

As a stop-gap, the content can stay on the Drupal server and the paths to it rewritten on output as absolute URLs so they can be accessed in the other learning environment (to which the export is imported).