fix shuffle button update when adding tracks to playlist. Closes #399

The root problem is that the jplayer playlist is not being notified when "shuffle" is activated/deactivated. 
I couldn't just add that notification, because the jplayer version used in the project has a bug that makes the player fail when the "shuffle" flag is true (check res/js/ext/jplayer.playlist.js, lines 475 and 503, where "playRandomTrack" is invoked, without referencing "this")