Rewrote the model theory for RDFS. Quite nice!

* [PRINT_EXT,EXT] Added the write_method/1 option to print_collection/2 for greater flexibility.
* [RDF_MT,NEW] Rewrote the model theoretic implementation of RDFS.
** [RDF_MT_PRINT,NEW] Module for printing the data model of the RDF model theory.
** [RDF_MT,NEW] The data model for the RDF model theory.
** [RDF_MT_I,NEW] The interpretation function.
** [RDF_MT_BUILD,NEW] Predicates fro building RDFS semantic models.
** [RDF_MT_TEST,NEW] Unit test for RDFS model theory. Still very incomplete.
* [RDF_TERM,NEW] Added rdf_iri/2, rdf_is_iri/1, rdf_plain_literal/[1,2], rdf_typed_literal/[1,2]