Fixed bug in RevoluteJointModel::distance() giving large negative numbers.

RevoluteJointModel::distance() was giving large negative values for a continuous joint, and upon investigation it turns out that there is nothing to enforce the continuous joint values to be between -pi and +pi or 0 and 2*pi.  The code in distance() was correct if the values *were* constrained, so I just added an extra "fmod()" call.