added ntriples writer

Added ntriples writer.
If you get an `Rdf#Node` in your hands, you need to make a choice based on what kind of node it is, and this is what `fold` is for. Extracting the information from the node and doing the control flow with if-then-else is **generally** a anti-pattern. I **really** want to avoid people writing code like that: `if (isURI(node)) doSomething(node.asInstanceOf[Rdf#URI])`, which I have seen a lot...

In any case, those functions have nothing to do in `RDFOps` anyway.