Define a reusable type for Creator/Contributor/Publisher

[mccalluc]

These all have essentially the same structure, so it'd be nice if they were built on the same base type. (#93 is looking just a vocabularies; #42 is also related.)

As it is, which each having different subelements, I need to write out each as a special case when getting the data out. (ex: I might just want all the names for indexing, and not care which parent element they are under.)

This would also prevent discrepancies like #82 where we expect the structures to be parallel, but they aren't.

Sorry if I'm missing something more subtle.