adding simple gripper action controller for single dof grippers

There could be an easier way to do this (using one of the effort controllers) but I could not find it so I implemented a simple version. This should service the GripperCommandAction that MoveIt! expects as well. So far, I have only tested in Gazebo with a PR2 gripper so it would be good to have this tried out on a real robot. 

The main difference for this action is limiting the max effort on a per call basis (which the other controllers are currently not setup to do).