prevent execution/failure of obsolete callbacks
prevent execution/failure of obsolete callbacks

callback() can modifie callbacks[keypath]
This will lead to an exception when callbacks unsubscribe other callbacks.
demo: http://jsfiddle.net/hpUTB/
QAPInt/rivets,re-clone/rivets,zongkelong/rivets,MishaMykhalyuk/rivets,GerHobbelt/rivets,mikeric/rivets,altmind/rivets,nopnop/rivets,benderTheCrime/tiny-rivets,mikeric/rivets,GerHobbelt/rivets,nopnop/rivets,MishaMykhalyuk/rivets,re-clone/rivets,re-clone/rivets,moneyadviceservice/rivets,zongkelong/rivets,nopnop/rivets,zongkelong/rivets,jccazeaux/rivets,QAPInt/rivets,QAPInt/rivets,GerHobbelt/rivets,altmind/rivets,MishaMykhalyuk/rivets,jccazeaux/rivets,altmind/rivets,mikeric/rivets,jccazeaux/rivets