implement and document Pipelines, see #3174

- heavily inspired by spray.io.Pipeline
- fully functional style: a stage returns the resulting commands and
  events, which makes it impossible to mess with the pipeline from the
  inside
- object allocations are optimized away for emtpy and 1-elem results
- added type-safety, verifying that stages match up
- management commands “from the side” for configuration or async events
- full Java API and docs

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

@@ -0,0 +1,45 @@
+/**
+ * Copyright (C) 2013 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.io.japi;
+
+//#message
+public class Message {
+  
+  static public class Person {
+    private final String first;
+    private final String last;
+
+    public Person(String first, String last) {
+      this.first = first;
+      this.last = last;
+    }
+
+    public String getFirst() {
+      return first;
+    }
+
+    public String getLast() {
+      return last;
+    }
+
+  }
+
+  private final Person[] persons;
+  private final double[] happinessCurve;
+
+  public Message(Person[] persons, double[] happinessCurve) {
+    this.persons = persons;
+    this.happinessCurve = happinessCurve;
+  }
+
+  public Person[] getPersons() {
+    return persons;
+  }
+
+  public double[] getHappinessCurve() {
+    return happinessCurve;
+  }
+}
+//#message