[ODE] AMotor small changes

[Russ Smith]

> I have done some changes in joint.cpp so that:
> * AMotor can now be used to attach a body to the static environment.
> * Angles passed to dJointSetAMotorParam (dParamLoStop,dParamHiStop)
> can now be outside the (-PI,PI) range.

i've checked in a modified version of these changes. the -pi..pi angle
change is not API backwards compatible, so i left it out. also, i added
code to handle the possibility of a reversed joint, i.e. dAttach
(0,body). please verify that this works for you.

russ.

-- 
Russell Smith
http://www.q12.org