def subscribe_parameter(param, &callback)
      sub = ParameterSubscriber.new(param, callback)
      @manager.add_parameter_subscriber(sub)
      sub
    end
def subscribe_parameter(param, &callback)
      sub = ParameterSubscriber.new(param, callback)
      @manager.add_parameter_subscriber(sub)
      sub
    end
subscribe to the parameter.

 @param [String] param name of parameter to subscribe
 @param [Proc] callback callback when parameter updated
 @return [ParameterSubscriber] created ParameterSubscriber instance