server playlist support

 * MPD commands implemented: listplaylists, listplaylist,
   listplaylistinfo
 * playlists now have an mtime property
 * fix player playlist method implementation issues
 * fix "last streamer disconnected" message for MPD clients which are
   not actually streaming
add MPD rename command
add MPD rm command
rewrite arg parsing and validation for groovebasin protocol

 * db delete commands are batched up during collection scanning
 * deleteTracks command deletes multiple files at once, emitting only
   one event for the delete operation
 * rewrite argument parsing and validation for groovebasin protocol.
   greatly enhanced security.