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/HasActorContext.java

@@ -0,0 +1,16 @@
+/**
+ * Copyright (C) 2013 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.io.japi;
+
+import akka.actor.ActorContext;
+import akka.io.PipelineContext;
+
+//#actor-context
+public interface HasActorContext extends PipelineContext {
+  
+  public ActorContext getContext();
+
+}
+//#actor-context