Fixes #2598, mousewheel event not firing on dom level 3 browsers

As described in #2598 I introduced a regression while updating the code to support the dom level 3 `wheel` event, since it wasn't add to the `Element.NativeEvents` map.
I've been reading about related issues. 
Both [in lighthouse](https://mootools.lighthouseapp.com/projects/2706/tickets/1015-mousewheel-should-follow-the-speed-of-the-wheel), [Stackoverflow](http://stackoverflow.com/q/5527601/2256325) and [a plugin](https://github.com/brandonaaron/jquery-mousewheel) related to delta(s).

@arian, @kentaromiura  what/how should we do here? 

should we use `wheel` to give just direction and make `delta[Y|X]` give a normalized value, or same as "our" wheel? there seems to not be a perfect solution that includes mouse acceleration.