Add methods generating & testing uniqueness of IDs

Add methods:
* self.generate_id - uses the integer_time_hash method to randomly generate an id; uses is_persisted? to check for uniqueness
* self.integer_time_hash - generates an integer from a time hash
* self.id_persisted? - constructs a full rdf_subject by creating a new instance for the ID and checking if it is persisted
* self.uri_persisted? - constructs a full rdf_subject by creating a new instance for the URI and checking if it is persisted