def add_subscriber(subscriber)
      uris = @master.register_subscriber(subscriber.topic_name,
                                         subscriber.topic_type.type)
      subscriber.set_manager(self)
      uris.each do |publisher_uri|
        subscriber.add_connection(publisher_uri)
      end
      @subscribers.push(subscriber)
      subscriber
    end
def add_subscriber(subscriber)
      uris = @master.register_subscriber(subscriber.topic_name,
                                         subscriber.topic_type.type)
      subscriber.set_manager(self)
      uris.each do |publisher_uri|
        subscriber.add_connection(publisher_uri)
      end
      @subscribers.push(subscriber)
      subscriber
    end
register a subscriber to master. raise if fail.
 @param [Subscriber] subscriber Subscriber to be added
 @return [Subscriber] subscriber