Support RDF data in PUT requests (see #23)

Specify RDF serialization in Content-Type header of the request.
Data is internally loaded into an RDF model, serialized as compact
JSON-LD with expanded arrays, and stored in Elasticsearch. See API
documentation page for supported content types and sample request.
Move JSON-LD creation and indexing code, set up local index tests

See #14. This change moves the JSON-LD conversion and indexing code
from oer-data to oer-api, and uses it to create a local
Elasticsearch index from some sample data. Enable existing but
previously ignored tests, which now run against this local index.