Implement Vocabulary::Term as subclass of URI used for vocabulary terms. This now implements Vocabulary::Term#label and Vocbulary::Term#comment in addition to Vocabulary.label_for and Vocabulary.comment_for.
Vocabulary::Term is also Enumerable, to generate statements based on the ruby vocabulary definition.
This resolves issue #159 and partially addresses #156.
Implement Vocabulary.to_graph to generate RDF statements for a ruby Vocabulary definition.