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/java/code/docs/actor/japi/Greeter.java

@@ -0,0 +1,26 @@
+/**
+ * Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.actor.japi;
+
+import akka.actor.UntypedActor;
+import java.io.Serializable;
+
+//#greeter
+public class Greeter extends UntypedActor {
+  
+  public static enum Msg {
+    GREET, DONE;
+  }
+  
+  @Override
+  public void onReceive(Object msg) {
+    if (msg == Msg.GREET) {
+      System.out.println("Hello World!");
+      getSender().tell(Msg.DONE, getSelf());
+    } else unhandled(msg);
+  }
+  
+}
+//#greeter