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/rst/modules/code/docs/actor/mailbox/DurableMailboxDocTestBase.java

@@ -0,0 +1,50 @@
+/**
+ * Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+package docs.actor.mailbox;
+
+//#imports
+import akka.actor.Props;
+import akka.actor.ActorRef;
+
+//#imports
+
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+import akka.testkit.AkkaSpec;
+import com.typesafe.config.ConfigFactory;
+import akka.actor.ActorSystem;
+import akka.actor.UntypedActor;
+
+public class DurableMailboxDocTestBase {
+
+  ActorSystem system;
+
+  @Before
+  public void setUp() {
+    system = ActorSystem.create("MySystem",
+        ConfigFactory.parseString(DurableMailboxDocSpec.config()).withFallback(AkkaSpec.testConf()));
+  }
+
+  @After
+  public void tearDown() {
+    system.shutdown();
+  }
+
+  @Test
+  public void configDefinedDispatcher() {
+    //#dispatcher-config-use
+    ActorRef myActor = system.actorOf(new Props(MyUntypedActor.class).
+        withDispatcher("my-dispatcher"), "myactor");
+    //#dispatcher-config-use
+    myActor.tell("test", null);
+  }
+
+  public static class MyUntypedActor extends UntypedActor {
+    public void onReceive(Object message) {
+    }
+  }
+
+}