switch to dispatcher-scoped settings for durable mailboxes, see #1836

- also switch SBT settings to enable testing of durable mailboxes
  centrally (if so desired, just uncomment testMailbox line in build.sbt)
- automatically start mongod, beanstalkd or redis-server when running
  the respective tests (assumes that the binaries are in PATH)
- unify settings extraction from dispatcher config, sub-scoping by
  mailbox type name

build.sbt

@@ -5,3 +5,5 @@
   (externalResolvers in LsKeys.lsync) := Seq("Akka Repository" at "http://akka.io/repository/")
 
   (description in LsKeys.lsync) := "Akka is the platform for the next generation of event-driven, scalable and fault-tolerant architectures on the JVM."
+
+  //testMailbox in Global := true