SPARQLStore.contexts() can now find empty named graphs

(if supported by the SPARQL endpoint)
The named graph query was polluted by the non-reset default-graph-uri parameter
Basic graph awareness for the SPARQLStore (default graph treated as a named graph)
The dataset default graph is now the default graph of the SPARQL endpoint
Merge pull request #402 from bcogrel/sparqlstore-graph-awareness

SPARQLStore graph awareness