add akka.Main and use that to add Hello World docs

- akka.Main will start an ActorSystem and one actor; when that actor
  terminates the system is shut down
- HelloWorld sample with two actors

akka-docs/rst/scala/code/docs/actor/IntroDocSpec.scala

@@ -0,0 +1,50 @@
+/**
+ * Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.actor
+
+import akka.testkit.AkkaSpec
+
+//#hello-world
+import akka.actor.Actor
+import akka.actor.Props
+
+class HelloWorld extends Actor {
+
+  override def preStart(): Unit = {
+    // create the greeter actor
+    val greeter = context.actorOf(Props[Greeter], "greeter")
+    // tell it to perform the greeting
+    greeter ! Greeter.Greet
+  }
+
+  def receive = {
+    // when the greeter is done, stop this actor and with it the application
+    case Greeter.Done ⇒ context.stop(self)
+  }
+}
+//#hello-world
+
+//#greeter
+object Greeter {
+  case object Greet
+  case object Done
+}
+
+class Greeter extends Actor {
+  def receive = {
+    case Greeter.Greet ⇒
+      println("Hello World!")
+      sender ! Greeter.Done
+  }
+}
+//#greeter
+
+class IntroDocSpec extends AkkaSpec {
+
+  "demonstrate HelloWorld" in {
+    expectTerminated(watch(system.actorOf(Props[HelloWorld])))
+  }
+
+}