Added script to stop the current motion trajectory

[MatthewPeterKelly]

This commit adds a script that stops an active motion trajectory. This is useful when the motion controller is running a long trajectory that needs to be canceled.

I also made small modifications to the MotionTrajectory and MotionControllerActionClient classes to make the stop() methods behave more like the send() methods.

[IanTheEngineer]

@rethink-forrest @MatthewPeterKelly this should not be merged until i have created the 5.2 feature branches

[MatthewPeterKelly]

This script should also work for the 5.2 branch, although I've not tested it.