Change callback signature for Model "update" event

I'm not sure this is exactly how the parameters should be named. The important fact is that the first argument is actually an array of [key, value]

README.markdown

@@ -239,7 +239,7 @@ Get a property.
 
 Set a property.
 
-#### on('update', function (key, value, source))
+#### on('update', function ([key, value], source, updateId))
 
 Emmitted when a property changes. 
 If `source !== this.id`