implement ResumeWriting, see #3200

also included:
- a complete rewrite of the TCP docs based on real/tested/working code
  samples
- an EchoServer implementation which handles all the edge cases,
  available in Java & Scala
- renamed StopReading to SuspendReading to match up with ResumeReading
- addition of Inbox.watch()
- Inbox RST docs for Java(!) and Scala

not included:
- ScalaDoc / JavaDoc for all IO stuff

akka-docs/rst/java/code/docs/io/japi/EchoServer.java

@@ -0,0 +1,35 @@
+/**
+ * Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.io.japi;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+
+import akka.actor.ActorRef;
+import akka.actor.ActorSystem;
+import akka.actor.Props;
+
+import com.typesafe.config.Config;
+import com.typesafe.config.ConfigFactory;
+
+public class EchoServer {
+
+  public static void main(String[] args) throws InterruptedException {
+    final Config config = ConfigFactory.parseString("akka.loglevel=DEBUG");
+    final ActorSystem system = ActorSystem.create("EchoServer", config);
+    try {
+      final CountDownLatch latch = new CountDownLatch(1);
+      final ActorRef watcher = system.actorOf(Props.create(Watcher.class, latch), "watcher");
+      final ActorRef nackServer = system.actorOf(Props.create(EchoManager.class, EchoHandler.class), "nack");
+      final ActorRef ackServer = system.actorOf(Props.create(EchoManager.class, SimpleEchoHandler.class), "ack");
+      watcher.tell(nackServer, null);
+      watcher.tell(ackServer, null);
+      latch.await(10, TimeUnit.MINUTES);
+    } finally {
+      system.shutdown();
+    }
+  }
+
+}