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.
you could add a check for `GamepadButton`:

```js
'GamepadButton' in window && button instanceof GamepadButton
```

or you could change it to this:

```js
typeof(button) === 'object' && button.value
```