Added gamepad axes support: ...

Great to see Gamepad support in the master branch! One of very few things stopping this emulator from hitting it big if you ask me. Howeever i think you missed the "axes" part described here: 

https://dvcs.w3.org/hg/gamepad/raw-file/default/gamepad.html#widl-Gamepad-axes

Without adding them i couldnt use the D-pad at all.