Added clamping to URDF effort and velocity limits.

The effort and velocity values in the effort-based joint position and velocity controllers now obey the effort and velocity limits specified in URDF files.

Without these changes, using a JointVelocityController with an apparently small derivative gain, such as 0.01, resulted in large effort values being sent to Gazebo. If Gazebo 1.9.1 received such a large effort value, it moved many of the model's links to the world's origin, making the model unusable. As long as reasonable effort limits are specified in the URDF file, the changes I have made to the position and velocity controllers prevent PID gains from resulting in an unusable Gazebo model.