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/Sample.scala

@@ -0,0 +1,24 @@
+/**
+ *  Copyright (C) 2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.duration
+
+object Scala {
+  //#dsl
+  import scala.concurrent.util.duration._ // notice the small d
+
+  val fivesec = 5.seconds
+  val threemillis = 3.millis
+  val diff = fivesec - threemillis
+  assert(diff < fivesec)
+  val fourmillis = threemillis * 4 / 3 // though you cannot write it the other way around
+  val n = threemillis / (1 millisecond)
+  //#dsl
+
+  //#deadline
+  val deadline = 10.seconds.fromNow
+  // do something
+  val rest = deadline.timeLeft
+  //#deadline
+}