Added debug util, use like this:

import org.remotefutures.util.Debug._

  // switches debugging on and off
  implicit final val DBG = true

  printDbg("my debug message gets only printed if DBG is true")
Moved PromiseCompletingRunnable in separate file
in order to share it amongst different impl.
* Changed security level to private[package]
* Replaced println with printDBG
Merge remote-tracking branch 'origin/master'
Merge pull request #21 from DistributedRemoteFutures/marvin-testing

Ya. Good!