added ntriples writer

Added ntriples writer.
Though in  my opinion parsers should be completely independent of the RDF. I should be able to parse a turtle file using the Jena Parser to the `Sesame#Triple`s and the Sesame parsers to produce `Jena#Triple`s . All a parser should do is have to call some trait with a method for adding(subject, predicate, object, resource) and you'd  be gone. Have not had time to look at that yet.