First stab at DepthData interface and CanvasRenderingContext2D extensions.

I changed ArrayBufferView into Uint16Array type.

I was thinking that for future-proofing (e.g. to allow Uint32Array usage in the future, as suggested by Benjamin at http://lists.w3.org/Archives/Public/public-media-capture/2014Aug/0102.html) we'd use ArrayBufferView similarly to e.g. WebSocket's send() does. But I think it makes the spec simpler if we use the Uint16Array type instead.

We can overload the constructor in future revisions of the spec, if the need arises.