Add charCode method binding to KeyboardEvent

The thing is, that even though keypress event is to be deprecated in favor of [beforeinput](http://www.w3.org/TR/DOM-Level-3-Events/#event-type-keypress), it's still in far future.

Currently there is no way to get hold of keypress event unicode value in Gecko based browsers where you can find it with certainty in charCode only. (based on my todays investigations).

For further reading, please see this [google groups thread](https://groups.google.com/d/msg/scala-js/NFdMIbLLupE/VqQd3aeUntMJ)