use unprefixed Gamepad API if available for WebKit (fixes #23)
Merge pull request #24 from cvan/unprefixedWebKit

use unprefixed Gamepad API if available for WebKit (fixes #23)
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.