Updates button getter for new GamepadButton API

Modern Firefox has aligned with the W3C Gamepad API spec, which calls for `gamepad.buttons` to be an array of GamepadButton objects rather than button state values. This adds support for that API. Tested in Firefox 29 for Linux.

I'm working on a patch to correctly map the PS4 Dualshock 4 controller, too, just need to make sure it behaves correctly on all platforms.
also, do you think this should this return `1.0` if `button.pressed === true`?