rethink CD metadata handling.

[kohlhase]

There are a couple of suggestions on how to handle this better than now, most from a LOD perspective:

• (CD License) CDLicense Tag? OM3#18
• CDComment: Clarify the semantics of CDComment OM3#71
• author, rights, derived-from Author, rights, and derived-from OM3#12
• (DC Metadata) Incorporate Dublin Core and Creative Commons instead of idiosyncratic metadata OM3#38
• URI metadata Allow for arbitrary metadata referenced by URI OM3#39