Adds mapping for Dualshock 4 controller in Firefox.

Chromium seems to directly support the PS4 Dualshock 4 controller,
but Firefox has some really strange mappings. This adds correct
mappings for the DS4 controller under Firefox.

Tested in Firefox 29 for Linux.
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.