Crawler.scala

override def receive: Receive = {
  case Start(start) =>
    crawlUrl(start)

  case CrawlResult(url, links) =>
    inProgress -= url

    links.foreach { link =>
      crawlUrl(link)
      referenceCount = referenceCount.updated(link.host, 
                                              referenceCount.getOrElse(link.host, 0) + 1)
    }

    if (inProgress.isEmpty) {
      result.success(referenceCount)
      context.stop(self)
    }
}