Added a new 'velocity_scaling_factor' parameter to joint_limits.yaml

This evenly reduces max joint velocity for all joints. It is particularly useful if you are using the default max_velocities from the URDF, but want to slow down your robot say by 10% (set it to 0.1)

Also, I added much needed documentation for how joint_limits.yaml works, inline.

As we discussed @mmurooka

PR follows https://github.com/ros-planning/moveit_ros/pull/486