We should have a way to get gamepad state asynchronously

[Manishearth]

Ideally, make it a variant of future_frame_data that gets you both the frame data and gamepads.

Currently we only have one async implementor of future_frame_data (GLWindow) and that has no gamepads, so it's not 100% necessary yet, but we may need it in the future.

cc @asajeffrey