Offer experiment results in a sparql endpoint

GOAL: As a user I want to use http://somurl/gerbil/sparql to retrieve for example the best annotator for microposts in a a2w task.

Therefore implement a SPARQL interface within gerbil so the sparql endpoint is bound to the gerbil webapp

Start by locating the GERBIL webapp entry point and the existing experiment-result handling; the issue names no files or tests. Determine how the /gerbil/sparql endpoint should be bound and what result data it must expose, then verify that a SPARQL client can retrieve the requested annotator results.

Written by the indexing model from the issue text.