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])))
+  }
+
+}

akka-docs/rst/scala/hello-world.rst

@@ -0,0 +1,44 @@
+##########################
+The Obligatory Hello World
+##########################
+
+Since every programming paradigm needs to solve the tough problem of printing a
+well-known greeting to the console we’ll introduce you to the actor-based
+version.
+
+.. includecode:: ../scala/code/docs/actor/IntroDocSpec.scala#hello-world
+
+The ``HelloWorld`` actor is the application’s “main” class; when it terminates
+the application will shut down—more on that later. The main business logic
+happens in the :meth:`preStart` method, where a ``Greeter`` actor is created
+and instructed to issue that greeting we crave for. When the greeter is done it
+will tell us so by sending back a message, and when that message has been
+received it will be passed into the behavior described by the :meth:`receive`
+method where we can conclude the demonstration by stopping the ``HelloWorld``
+actor. You will be very curious to see how the ``Greeter`` actor performs the
+actual task:
+
+.. includecode:: ../scala/code/docs/actor/IntroDocSpec.scala#greeter
+
+This is extremely simple now: after its creation this actor will not do
+anything until someone sends it a message, and if that happens to be an
+invitation to greet the world then the ``Greeter`` complies and informs the
+requester that the deed has been done.
+
+As a Scala developer you will probably want to tell us that there is no
+``main(Array[String])`` method anywhere in these classes, so how do we run this
+program? The answer is that the appropriate :meth:`main` method is implemented
+in the generic launcher class :class:`akka.Main` which expects only one command
+line argument: the class name of the application’s main actor. This main method
+will then create the infrastructure needed for running the actors, start the
+given main actor and arrange for the whole application to shut down once the
+main actor terminates. Thus you will be able to run the above code with a
+command similar to the following::
+
+  java -classpath <all those JARs> akka.Main com.example.HelloWorld
+
+This conveniently assumes placement of the above class definitions in package
+``com.example`` and it further assumes that you have the required JAR files for
+``scala-library`` and ``akka-actor`` available. The easiest would be to manage
+these dependencies with a build tool, see :ref:`build-tool`.
+