Update examples

[toliver]

Update example https://github.com/shadow-robot/sr-ronex/wiki/Control-a-Joint-Based-Robot-with-RoNeX

This example is using a controller of type robot_mechanism_controllers/JointPositionController. I think this type is not usable now that the drivers rely on ros_ethercat.

We should build a generic position controller similar to https://github.com/ros-controls/ros_controllers/blob/indigo-devel/velocity_controllers/src/joint_position_controller.cpp but instead of a velocity command, the output should be an effort command.