Akka Actor that tracks actors for shutting down the system when they all are terminated. @see http://letitcrash.com/post/30165507578/shutdown-patterns-in-akka-2

Reaper.scala

import akka.actor.{ActorLogging, Terminated, ActorRef, Actor}
import scala.collection.mutable

object Reaper {
  case class WatchMe(ref: ActorRef)
}

class Reaper extends Actor with ActorLogging {
  import Reaper._

  val souls = new mutable.ListBuffer[ActorRef]()

  def receive = {
    case WatchMe(ref) =>
      context.watch(ref)
      souls += ref
    case Terminated(ref) =>
      souls -= ref
      if (souls.isEmpty) allSoulsReaped()
  }

  def allSoulsReaped() {
    log.warning("All Souls Reaped! System shutting down...")
    context.system.shutdown()
  }

}