add matchTarget to event forwarded by on()

See https://bugs.dojotoolkit.org/ticket/18355
I I explained on the bugtracker, that was to keep the event object consistent instead of having a special case for event delegation.
This way, when using hitch, developers can always rely on matchTarget without taking care if it's a delegated event or not. It seems easier too me (from the a dojo user point of view).

But that can be changed if you don't agree.