introducing: MainBus feat. LoggingBus

most tests passing, everything compiling, but docs not updated and nasty
thread-leak preventing me from running the whole test-suite (which is
the reason for this commit: I want to chase down that one first).

- the app.mainbus is classified by Class[_] (currently lookup, will
  possibly change to sub-class-aware) and accepts AnyRef messages
- LoggingBus handles akka.event-handlers from config specially:
  + start them as system services, supervised by SystemGuardian
  + keep their subscriptions in sync when logLevel_= is called
  + send them InitializeLogger(bus) message before subscribing them (so
    they can register for extras like Mute/UnMute)
- two-phased start-up: first phase with actor-less stdout logging, then
  subscription of config loggers, then remove stdout logger (logLevels
  configurable separately)
- MainBusReaper watches registered receivers and unsubscribes them upon
  death (started in phase 2)
- logger factory on Logging object, needs app/bus and log source;
  default instance in app.log

akka-actor/src/main/scala/akka/dispatch/AbstractDispatcher.scala

@@ -6,7 +6,7 @@ package akka.dispatch
 
 import java.util.concurrent._
 import java.util.concurrent.atomic.AtomicLong
-import akka.event.EventHandler
+import akka.event.Logging.Error
 import akka.config.Configuration
 import akka.util.{ Duration, Switch, ReentrantGuard }
 import java.util.concurrent.ThreadPoolExecutor.{ AbortPolicy, CallerRunsPolicy, DiscardOldestPolicy, DiscardPolicy }
@@ -66,7 +66,7 @@ final case class TaskInvocation(app: AkkaApplication, function: () ⇒ Unit, cle
     try {
       function()
     } catch {
-      case e ⇒ app.eventHandler.error(e, this, e.getMessage)
+      case e ⇒ app.mainbus.publish(Error(e, this, e.getMessage))
     } finally {
       cleanup()
     }