Updated documentation of Actors (Java). See #1435

* Aligned the Java and Scala documentation for Actors
* Implemented hotswap samples in Java, and documented in same way as Scala docs
* Improved Actors (Scala) docs
* Fixed wrong preRestart and postRestart in UntypedActor
* Changed name of Dispatchers.fromConfig to newFromConfig and made it Java friendly
* Added ActorRef.ask with Timeout parameter in addition to the timeoutMillis

akka-docs/java/code/akka/docs/actor/ImmutableMessage.java

@@ -0,0 +1,25 @@
+package akka.docs.actor;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+//#immutable-message
+public class ImmutableMessage {
+  private final int sequenceNumber;
+  private final List<String> values;
+
+  public ImmutableMessage(int sequenceNumber, List<String> values) {
+    this.sequenceNumber = sequenceNumber;
+    this.values = Collections.unmodifiableList(new ArrayList<String>(values));
+  }
+
+  public int getSequenceNumber() {
+    return sequenceNumber;
+  }
+
+  public List<String> getValues() {
+    return values;
+  }
+}
+//#immutable-message