Adds support for translate3d transform

For browsers that support translate3d transforms. This will improve DOM performance slightly (mostly for mobile/tablet non-accelerated web apps) in desktop builds.

Adds a :position(x, y) method that abstracts deciding whether to "top/left" or "translate3d". Falls back to "top/left" if there is no support for translate3d.

Note: checking for style.Perspective only returns true if the browser supports unprefixed transform. Tested on webkit browsers.