RDF endpoints

The old CRUD (**c**reate, **r**ead, **u**pdate, **d**elete) 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.