[Playlist] criteria nodes

[Lucas Gonze]

Reading the spec on the criteria node at
http://playlist.musicbrainz.org/playlist/moin.cgi/PlaylistFormat

I want to point out the alternative, which is to use xpath expressions 
on metadata.  This would allow us to avoid defining a way to do 
something that has already been done elsewhere.

To do this in practice, the criteria node would have a definition 
something like:
The Criteria node specifies an xpath expression describing all nodes to 
select, with the expression to be applied to all songs available to the 
operator of the query.

- Lucas