Document the button names

[cvan]

I was confused about FACE_1, FACE_2, FACE_3, and FACE_4 until I read http://www.html5rocks.com/en/tutorials/doodles/gamepad/ and used http://www.html5rocks.com/en/tutorials/doodles/gamepad/gamepad-tester/tester.html

I'm still unsure of SELECT_BACK and START_FORWARD. The aforementioned tutorial refers to SELECT and START only; what would back and forward do on particular controllers?

And what should HOME do (I thought this would be similar to Select, but I'm probably just unfamiliar with controllers that have Home buttons).