added ntriples writer
move ntriples to io folder, made fixes according to peer reviews, fixed overrides
memory reduction writing to OutputStream & fix eol
Merge pull request #1 from read-write-web/topic/ntripleswriter

memory reduction writing to OutputStream & fix EOL
Merge pull request #209 from antonkulaga/ntripleswriter

Added RDFWriter[Rdf, Try, NTriples].