Fix 2598 - mousewheel dom3 and wheel property

This pull requests fixes 
- the mousewheel event in Browsers using DOM3 `wheel` event
- the `.wheel` property of MooTools's DOMEvent which would otherwise be broken in Firefox.

It also updates the normalize of the `e.wheel` value according to [each browsers return](https://docs.google.com/spreadsheets/d/1Gf41PhtPXZuwPGjh8Sc52s1JBd7VFPhpXtbbxLuLbjw/edit#gid=0) on `event.wheelDelta`, `event.detail` and `event.deltaY`.

The reason why `deltaY` is mixed in here is because Firefox returns `undefined` for `event.wheelDelta` and `0` for `event.detail`, as can be tested in this jsFiddle: http://jsfiddle.net/mNdwW/

Tested [in SauceLabs](https://travis-ci.org/SergioCrisostomo/mootools-core/builds/29683242) and [manually](http://jsfiddle.net/75JJU/) on IE7-11, Opera (latest) Win & Mac, Safari 7, Chrome (latest) Win & Mac, Firefox (latest) Win & Mac

fixes #2598, closes #2599, closes #2620
Just use multiple `var`s, looks less awkward with the callback.
Also you forgot the `;`