Add Gamepad Support

I had my xbox 360 controller sitting on my desk and a couple of free hours so I thought - wouldn't it be great to play 2048 with my gamepad?

So far it's only works on Chrome. I got started on Firefox support, and that got boring fast and I've run out of time to waste this afternoon.

D pad & left analog stick work (but are maybe a little sensitive?), and if you're in game over mode, any button you mash will restart.

You can test it out at http://joho.github.io/2048/ I've played a few times and it appears to work fine.