See #70: Execute handler on original event in listen-live.

As stated in #70, Currently, the original event is discarded, and the event handler is run on a new `goog.events.Event` with the same target property. This makes `.preventDefault` and `.stopPropagation` impossible.

With this commit, `listen-live` will execute the handler on the original event object, and then return a new event. This strikes a compromise between immutability and the need to cause side effects.