added ntriples writer

Added ntriples writer.
There is also filter. Here is a simple example from a little project I am working on [rww-scala-js](https://github.com/read-write-web/rww-scala-js/blob/master/src/main/scala/rww/ui/foaf/Person.scala) where you want to get the pictures on a person:

```scala
val l = (P / foaf.depiction).toList
        l.filter { p =>
            p.pointer.fold(uri => true, bn => false, lit => false)
        }.headOption.map(_.pointer.toString).getOrElse("img/avatar.png")
      }
```

this would be much clearer written like this:

```scala
val l = (P / foaf.depiction).toList
        l.filter( _.pointer.isURI  ).headOption.map(_.pointer.toString).getOrElse("img/avatar.png")
      }
```