add preprocessor for RST docs, see #2461 and #2431

The idea is to filter the sources, replacing @<var>@ occurrences with
the mapping for <var> (which is currently hard-coded). @@ -> @. In order
to make this work, I had to move the doc sources one directory down
(into akka-docs/rst) so that the filtered result could be in a sibling
directory so that relative links (to _sphinx plugins or real code) would
continue to work.

While I was at it I also changed it so that WARNINGs and ERRORs are not
swallowed into the debug dump anymore but printed at [warn] level
(minimum).

One piece of fallout is that the (online) html build is now run after
the normal one, not in parallel.

akka-docs/scala/code/docs/actor/UnnestedReceives.scala

@@ -1,50 +0,0 @@
-/**
- * Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
- */
-package docs.actor
-
-import akka.actor._
-import scala.collection.mutable.ListBuffer
-
-/**
- * Requirements are as follows:
- * The first thing the actor needs to do, is to subscribe to a channel of events,
- * Then it must replay (process) all "old" events
- * Then it has to wait for a GoAhead signal to begin processing the new events
- * It mustn't "miss" events that happen between catching up with the old events and getting the GoAhead signal
- */
-class UnnestedReceives extends Actor {
-  import context.become
-  //If you need to store sender/senderFuture you can change it to ListBuffer[(Any, Channel)]
-  val queue = new ListBuffer[Any]()
-
-  //This message processes a message/event
-  def process(msg: Any): Unit = println("processing: " + msg)
-  //This method subscribes the actor to the event bus
-  def subscribe() {} //Your external stuff
-  //This method retrieves all prior messages/events
-  def allOldMessages() = List()
-
-  override def preStart {
-    //We override preStart to be sure that the first message the actor gets is
-    //'Replay, that message will start to be processed _after_ the actor is started
-    self ! 'Replay
-    //Then we subscribe to the stream of messages/events
-    subscribe()
-  }
-
-  def receive = {
-    case 'Replay ⇒ //Our first message should be a 'Replay message, all others are invalid
-      allOldMessages() foreach process //Process all old messages/events
-      become { //Switch behavior to look for the GoAhead signal
-        case 'GoAhead ⇒ //When we get the GoAhead signal we process all our buffered messages/events
-          queue foreach process
-          queue.clear
-          become { //Then we change behaviour to process incoming messages/events as they arrive
-            case msg ⇒ process(msg)
-          }
-        case msg ⇒ //While we haven't gotten the GoAhead signal, buffer all incoming messages
-          queue += msg //Here you have full control, you can handle overflow etc
-      }
-  }
-}