Improvements to SPARQL querying and HTML descriptions of resources.

* [DBPEDIA,SC] Simplified this module, relocating code to SPARQL_EXT and making it more generic.
* [GRAPH_CLOSURE,NEW] New module that allows efficient calculation of relational closures. Currently used to close SPARQL queries under owl:sameAs / identity.
* [RDF_DESCRIBE_WEB,NEW] New module generating HTML descriptions of RDF resources. Just a simple table for now.
* [RDF_WEB,NEW] rdf_web_argument/2 converts between user input and Prolog RDF terms.
* [RFC2616_ACCEPT_LANGUAGE,EXT] The list of language tags is now ordered by quality value.
* [RFC2616_ACCEPT_LANGUAGE,NEW] Added 'phrase_Accept-Language'/2 which returns the total order of language tags for a given HTTP request.
* [SPARQL_EXT,NEW] Added predicates for finding, asserting, and describing resources using SPARQL. Added support for identity closures as well.
* [SPARQL_SAMEAS,SC] Removed this module. Now integrated with SPARQL_EXT as an option.