Adding conventional PID computation, deprecating current PID computation

This PR adds a member function with to signatures `Pid::setError()` and adds deprecation warnings to the current `Pid::updatePid()` member functions, along with appropriate documentation.

This new member function is identical to the deprecated functions, except that it implements the conventional  sign of the input to a PID controller (error = target - current) discussed in this issue: https://github.com/willowgarage/ros_controllers/issues/2