code: def announce(key, port, opts={}, &block)
        payload = [key,port]
        link.send 'announce', payload, opts, &block
        if config.auto_announce
          periodically(config.auto_announce_interval) do
            link.send 'announce', payload, opts, &block
          end
        end
      end
docs: Announce a specific service `key` available on specific `port`
   passed block is called when the announce is sent
   @param key [string] service identifier
   @param port [int] service port number
   @block callback