Add scale property to TouchEvents

This makes it easier to recognize pinch to zoom gesture on JS side.

The scale property is non-standard but Apple implements in Mobile Safari, see
http://developer.apple.com/library/safari/#documentation/UserExperience/Reference/TouchEventClassReference/TouchEvent/TouchEvent.html#//apple_ref/doc/uid/TP40009358

What do you think? This pull request might not be mergeable as is, it's more like an opener for discussion.