Fixed PID destructor bug, cleaned up code

Fixed issue where the control_toolbox::Pid class was destructed during initialization [here](https://github.com/davetcoleman/ros_controllers/compare/hydro-devel#L3L80):
```
control_toolbox::Pid pid;
``` 

Also lots of documentation and cleanup.

This PR is required for https://github.com/ros-controls/control_toolbox/pull/1 to work properly