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/Watcher.java

@@ -0,0 +1,34 @@
+package docs.io.japi;
+
+import java.util.concurrent.CountDownLatch;
+
+import akka.actor.ActorRef;
+import akka.actor.Terminated;
+import akka.actor.UntypedActor;
+
+public class Watcher extends UntypedActor {
+  
+  static public class Watch {
+    final ActorRef target;
+    public Watch(ActorRef target) {
+      this.target = target;
+    }
+  }
+  
+  final CountDownLatch latch;
+
+  public Watcher(CountDownLatch latch) {
+    this.latch = latch;
+  }
+
+  @Override
+  public void onReceive(Object msg) throws Exception {
+    if (msg instanceof Watch) {
+      getContext().watch(((Watch) msg).target);
+    } else if (msg instanceof Terminated) {
+      latch.countDown();
+      if (latch.getCount() == 0) getContext().stop(getSelf());
+    }
+  }
+
+}