use unprefixed Gamepad API if available for WebKit (fixes #23)
Fixes regression in `WebKitPlatform.factory`.

Some `gamepadGetter` functions were returning the browser's gamepad
inquiry function instead of the gamepad list returned by that function.
Adds regression tests and fixes the issue.
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.