Add rdf:langString from RDF 1.1  to OWL/RDF vocabulary.

Make BUILT_IN_ANNOTATION_PROPERTY_IRIS unmodifiable

Add Nonnull annotations.
Switch comments to correct types
Merge pull request #178 from sesuncedu/add_rdf_langString_to_vocab

Add rdf:langString from RDF 1.1  to OWL/RDF vocabulary.