ADDED: rdf_assert2/4 which succeeds with uninstantiated graph argument.
EXTENDED: Added several RDF prefix registrations.
IMPROVED: Optional graph argument in:
            - rdf_assert_instance/3
            - rdf_assert_property/2
            - rdf_assert_list/3
            - rdf_assert_datatype/5
            - rdf_assert_literal/[5,6]
IMPROVED: rdf_create_next_resource/5 now sets the instance class
          (optional) and graph (also optional).
IMPROVED: Optional superclass argument in:
            - rdfs_assert_subclass/3
            - rdfs_assert_class/5
CLEANUP: Removed rdf_remove_property/2.
CLEANUP: Renamed rdf_remove_resource/2 to rdf_remove_term/2
         (since it is not closed under identity).