[Playlist] new RDF vocab

[Lucas Gonze]

I keep meaning to respond to this and getting bogged down in the
possibilities, so here's a short version: content resolution is indeed
a great use of semantic web technologies.  The only reservation I have
is wanting to be sure that the subtleties of XSPF semantics are being
accurately modeled in the RDF.  That's where I always felt that
Libby's RDF fell short.