RDF endpoints

[jgonggrijp]

The old CRUD (create, read, update, delete) interface for accessing collections, records and annotations is currently under the /api prefix. This structure already more or less echoes the namespaces as dicussed in #156, but the interfaces need to be converted to RDF using the mechanism from restframework-rdf. /api is a little bit awkward in an RDF term, so we should consider also moving these endpoints to a different prefix such as /rdf. This prefix needs to be a Django setting and must be kept consistent with the namespaces of our terms. Other endpoints that are currently routed under /api can stay there.

This is a general principle; we can create more specific issues for specific types of entities as we go.