Partial resolution #2 of: https://www.pivotaltracker.com/story/show/59243236

Renamed NamespaceContext -> NamespaceRdfContext for consistency
Widening types for RdfStream
Adding convenience methods to RdfStream to allow limiting and skipping RDF triples
Improvements to RdfStream type facilities
Moving iteration all the way into domain models (e.g. FedoraResource)
Pushed RDF mutation all the way through HTTP API, for replacement of properties
Just moving ManagedRdf to a more appropriate package