Gamepad-related improvements for JS target

As of this specification ( https://dvcs.w3.org/hg/gamepad/raw-file/default/gamepad.html ) adds:

Declarations for js.html.GamepadButton and js.html.GamepadEvent (Gecko-only, because only Mozilla implemented gamepads in the right way).
Adds missing parameters for js.html.Gamepad, such as .connected (returns whether the gamepad is connected to the device) and .mapping (indicating whether the browser has remapped the controls on the device to a known layout).
Changes the type of Gamepad.buttons to Array<Dynamic> to fit Gecko-powered browsers.