add Accessibility Object Model

[anpross]

would be mostly red but here are all the infos: http://wicg.github.io/aom/

[steveblue]

Can this please be added soon? AOM is now in WebKit nightly builds.

[Seirdy]

It looks like this should be broken up into many smaller features, such as ARIA attribute reflection.