SPARQL linked-data fragments-style API

[inaimathi]

Implement an LD-style interface for our knotation documents. We want to avoid going down to a triple-store this time (which should simplify things in terms of both querying pipeline and the amount of state we need to store). We were primarily using the triple store to support recursive queries (which LD interfaces specifically punt up to the client level), so this should be relatively straightforward.

[inaimathi]

Duplicate of #62