Add script execution with reporting to primary client

[urfeex]

Feature summary

As discussed in UniversalRobots/Universal_Robots_ROS_Driver#118 it would be good to have a script execution interface that knows when a script snippet has finished (or failed).

Adding the KeyMessage to the primary client and some logic around that should make it possible to add that kind of interface.

Tasks

To complete this issue involves

• Implement the feature
• Make documentation
• Make Unit test
• Make example
• Test on real hardware