Fixing handling of non-world frame velocities in setModelState.

For issue #240 
Part of the problem is that frame_pos is the pose of the relative_entity (e.g., the "chassis" link in the Pioneer model). This means that when we call model->SetWorldPose(target_pose) on line 1310, we'll setting the model's position using the position of the relative_entity. If relative_entity has a non-zero offset from the model origin, then we'll be repeatedly moving the model by the amount of the offset. In the case of the pioneer, the model has a Z offset of 0.16 meters, so every time this function is called, the robot climbs 16 centimeters into the air. If you call it with great enough frequency, the robot flies.