[gamepad] Add an event-based input mechanism

Just as I mentioned in my previous email to this list, I recently was asked
to review the Gamepad API draft specification.  My background is games
though I've done some scientific computing with alternate input devices too.

http://chadaustin.me/2014/10/the-gamepad-api/

I'd like to make a second proposed change to the gamepad API, an
event-based mechanism for receiving button and axis changes.

The full rationale is explained in the linked article, but the summary is:

1) an event-based API entirely avoids the issue of missed button presses
2) event-based APIs don't require non-animating web pages to use
requestAnimationFrame just to poll gamepad state
3) an event-based API could (and should!) give access to high-precision
event timing information for use in gesture recognition and scientific
computing
4) event-based APIs can reduce processing latency when a button changes
state rapidly in low-frame-rate situations

-- 
Chad Austin
http://chadaustin.me