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/java/code/docs/transactor/Counter.java

@@ -0,0 +1,28 @@
+/**
+ * Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.transactor;
+
+//#class
+import akka.transactor.*;
+import scala.concurrent.stm.Ref;
+import scala.concurrent.stm.japi.STM;
+
+public class Counter extends UntypedTransactor {
+    Ref.View<Integer> count = STM.newRef(0);
+
+    public void atomically(Object message) {
+        if (message instanceof Increment) {
+            STM.increment(count, 1);
+        }
+    }
+
+    @Override public boolean normally(Object message) {
+        if ("GetCount".equals(message)) {
+            getSender().tell(count.get(), getSelf());
+            return true;
+        } else return false;
+    }
+}
+//#class