Adding sparql service description for fcr:sparql GET requests.

https://www.pivotaltracker.com/s/projects/684825/stories/72619732

I've made the sparql service description output for a fcr:sparql GET request sticks to content negotiation machinery with the jena model serialization and RDFLanguages. I think it'll support all rdf triples format and the default is set for application/rdf+xml.
Sorry I have to include my last commit https://github.com/lsitu/fcrepo4/commit/22d6b27e29e341a5c2eb2f4cc324b5f8c8d5387c for PR https://github.com/fcrepo4/fcrepo4/pull/416 here since I need to reference that sparql sd namespace here.