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/common/code/docs/duration/Java.java

@@ -0,0 +1,27 @@
+/**
+ *  Copyright (C) 2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.duration;
+
+//#import
+import scala.concurrent.util.Duration;
+import scala.concurrent.util.Deadline;
+//#import
+
+class Java {
+  public void demo() {
+    //#dsl
+    final Duration fivesec = Duration.create(5, "seconds");
+    final Duration threemillis = Duration.parse("3 millis");
+    final Duration diff = fivesec.minus(threemillis);
+    assert diff.lt(fivesec);
+    assert Duration.Zero().lt(Duration.Inf());
+    //#dsl
+    //#deadline
+    final Deadline deadline = Duration.create(10, "seconds").fromNow();
+    final Duration rest = deadline.timeLeft();
+    //#deadline
+    rest.toString();
+  }
+}