Reverse the body arguments for PrismaticJoint and WheelJoint

[wnbittle]

Currently the body1 and body2 arguments for the PrismaticJoint and WheelJoint are not intuitive. The given axis get fixed to the second body making for some odd joint definitions. The desire is to change this so that the axis is fixed in the first body's coordinates. This would change:

WheelJoint wj = new WheelJoint(wheel, frame, ..., ...,);

to

WheelJoint wj = new WheelJoint(frame, wheel, ..., ...,);