def subscribe(topic_name, topic_type, &callback)
      sub = Subscriber.new(@node_name,
                           resolve_name(topic_name),
                           topic_type,
                           callback)
      @manager.add_subscriber(sub)
      trap_signals
      sub
    end
def subscribe(topic_name, topic_type, &callback)
      sub = Subscriber.new(@node_name,
                           resolve_name(topic_name),
                           topic_type,
                           callback)
      @manager.add_subscriber(sub)
      trap_signals
      sub
    end
start to subscribe a topic.

 @param [String] topic_name name of topic (string)
 @param [Class] topic_type Topic instance
 @return [Subscriber] created Subscriber instance