Gamepad support

Hi! I saw that gamepad support was something you may implement in the future. I couldn't wait for it and played around with it a little myself. This works on the current firefox if you enable gamepad support on about:config. If you like this i might add:

- [x] Firefox support
- [ ] Chrome support
- [ ] Support for the new spec implemented in Firefox 28
- [ ] User-mappable buttons
- [ ] Ability to save button configuration in local storage

I took the liberty to remap your buttons to suit my SNES controller :P