Update 2015-12-23

Changelog:

• Improved Mixed 2 level art.
• It's no longer possible to drive underneath the track on Mixed 2.
• Added a temporary workaround for glitched powertrain inertias.
• AI should now follow racing line better and thus stay on track.
• It's now possible to build a Class A European 1.
• It's now possible to build a Class A American 2.
• Added European 1 Class A AI opponents.
• Added American 2 Class A AI opponents.
• Retuned keyboard controller.
• Added a festive surprise :)