Add rdf:HTML and rdf:langString from RDF 1.1 concepts

RDF 1.1 Concepts and Abstract Syntax
W3C Last Call Working Draft 23 July 2013
http://www.w3.org/TR/2013/WD-rdf11-concepts-20130723/

http://www.w3.org/TR/2013/WD-rdf11-concepts-20130723/#section-html

http://www.w3.org/TR/2013/WD-rdf11-concepts-20130723/#section-Datatypes