Vehicle steering return-to-centre

Inspired by [Charles Fusner's update](http://www.garagegames.com/community/blogs/view/15923) to [James Jacoby's original resource](http://www.garagegames.com/community/resources/view/6305). These changes allow Vehicles to automatically centre their steering based on their current velocity. It makes vehicle handling much more realistic - and easier, especially with keyboard steering!

Recommended datablock settings for the Cheetah with this patch:

    powerSteering = true;
    steeringReturn = 1;
    steeringReturnSpeedScale = 0.5;