Playlist namespace URI

Current playlist namespace points to nowhere. Although this is not mandatory, we might want to provide description for our custom namespaces.

See http://xspf.org/ns/0/ for example.

<playlist xmlns="http://xspf.org/ns/0/" xmlns:vlc="http://www.videolan.org/vlc/playlist/ns/0/" version="1">

We might want to change URI to something more manageable:

http://www.videolan.org/ns/playlist/0/ or http://www.videolan.org/vlc/ns/playlist/0/