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/camel/Consumers.scala

@@ -1,73 +0,0 @@
-/**
- * Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
- */
-
-package docs.camel
-
-import language.postfixOps
-
-object Consumers {
-  object Sample1 {
-    //#Consumer1
-    import akka.camel.{ CamelMessage, Consumer }
-
-    class Consumer1 extends Consumer {
-      def endpointUri = "file:data/input/actor"
-
-      def receive = {
-        case msg: CamelMessage ⇒ println("received %s" format msg.bodyAs[String])
-      }
-    }
-    //#Consumer1
-  }
-  object Sample2 {
-    //#Consumer2
-    import akka.camel.{ CamelMessage, Consumer }
-
-    class Consumer2 extends Consumer {
-      def endpointUri = "jetty:http://localhost:8877/camel/default"
-
-      def receive = {
-        case msg: CamelMessage ⇒ sender ! ("Hello %s" format msg.bodyAs[String])
-      }
-    }
-    //#Consumer2
-  }
-  object Sample3 {
-    //#Consumer3
-    import akka.camel.{ CamelMessage, Consumer }
-    import akka.camel.Ack
-    import akka.actor.Status.Failure
-
-    class Consumer3 extends Consumer {
-      override def autoAck = false
-
-      def endpointUri = "jms:queue:test"
-
-      def receive = {
-        case msg: CamelMessage ⇒
-          sender ! Ack
-          // on success
-          // ..
-          val someException = new Exception("e1")
-          // on failure
-          sender ! Failure(someException)
-      }
-    }
-    //#Consumer3
-  }
-  object Sample4 {
-    //#Consumer4
-    import akka.camel.{ CamelMessage, Consumer }
-    import scala.concurrent.util.duration._
-
-    class Consumer4 extends Consumer {
-      def endpointUri = "jetty:http://localhost:8877/camel/default"
-      override def replyTimeout = 500 millis
-      def receive = {
-        case msg: CamelMessage ⇒ sender ! ("Hello %s" format msg.bodyAs[String])
-      }
-    }
-    //#Consumer4
-  }
-}