Add RDFTerm.ntriplesString, IRI.getIRIString, and BlankNode.internalIdentifier, Fixes #14

Contains one way of fixing issue #14, but specifying a concrete representation, such as RDF-1.1 N-Triples Canonical, as the way of serialising any RDFTerm to a String, while adding a specific way of serialising IRI to a raw, un-quoted, string. It also modifies the method name that we already had on BlankNode to further indicate its scope.