Automatically destroy removed instances

With multiple instances of jplayer, if one got removed from the dom without being destroyed, the others would cause a javascript error on play. This was caused by keeping a list of instances ($.jPlayer.prototype.instances) that would get out of sync with the page. With this patch, jPlayer clears the list of any nonexistent instances before iterating over the list.

The error was triggered from pauseOthers() and $.jPlayer.pause().