fixing torque application in btMultiBody

I found that I was unable to apply torques at joints (using btMultiBody::addJointTorque).
I traced the issue to btMultiBodyDynamicsWorld::solveConstraints, where clearForcesAndTorques is called before stepVelocities.
My tentative fix is to move the call to clearForcesAndTorques a few lines down, right after the call to stepVelocities.
My pull request includes the relevant fix to btMultiBodyDynamicsWorld.cpp, as well as a hacky test script (since I wasn't able to build any of the GUI test applications).