Feature/vocabs

This implements controlled vocabularies in RDF.  

There are a few things I would like to open as new tickets if this gets pulled:

1. The QA implementation needs some work. We need a working default RDF search and it's not necessarily clear to me how the full record requests are intended to be used.  Hopefully our needs for QA become more clear as we work on the form.

2. There's not currently a way to list the terms in a particular vocabulary. This would be nice to have for generating drop down menus.  This might involve a minor patch to RDF::Vocabulary and/or RDF::StrictVocabulary.  I've delayed on it while I think about whether that's the correct approach. 

Other than that, there's not a lot to this.  It creates subclasses of OD::RDF::RdfResource that restrict their URIs to a particular vocabulary, a way of registering vocabularies in configuration, an instance method to fetch data about an RdfResource from its rdf_subject URI, and a class method for pulling data about a vocabulary's terms en masse.

Comments welcome/practically demanded.