Small improvements to loading/saving RDF data, especially VoID-described RDF datasets.

* [RDF_NAMESPACE,EXT] Automatically load common namespaces.
* [RDF_SERIAL,NEW] Added rdf_guess_data_format/2, borrowed from ClioPatria.
* [SGML_PARSE,NEW] New module containing two predicates I bollow from ClioPatria for creating and removing SGML parsers.
* [VOID,SC] Removed the dependency on a ClioPatria library.
* [VOID,EXT] VoID graphs are only written to Turtle (this used to be Turtle and XML/RDF).
* [XML,NEW] Added xml_doctype/2, borrowed from ClioPatria.