Support running multiple S/C instances inside the same JVM and even with the same Akka instance

[pstutz]

Required steps:

• Make the actor system and its name configurable.
• Have different namespaces for the coordinator/worker/logger inside the same actor system.
• Get rid of ActorSystemRegistry and explicitly pass the reference to the constructors of the classes that need access to an actor system.

[pstutz]

This is now fixed on the scala-2.11-akka-2.3.2-migration branch, see test case @ https://github.com/uzh/signal-collect/blob/scala-2.11-akka-2.3.2-migration/src/test/scala/com/signalcollect/ActorSystemSpec.scala