on
Add event handlers to a list.
    list.on(eventType, handler)
  
  Listens to eventType on list and calls handler when the event is dispatched.  This is simply
a reference to can-event.on as all of can-event methods are available on DefineMap.
var list = new DefineList(["CanJS","StealJS"])
list.on("add", function(ev, added, index){ ... });
list.push("DoneJS");
  
  Parameters
- eventType 
{String}:The type of event to bind this handler to.
 - handler 
{function(event, args...)}:The handler to be called when this type of event fires The signature of the handler depends on the type of event being bound.