+act #2075 Added possibility to pass an ExecutionContext to an ActorSystem.

This PR adds the possibility to pass an ExecutionContext to an ActorSystem. This ExecutionContext will be used in the default dispatcher, unless the executor of the default-dispatcher is being overridden in the application.conf. If no ExecutionContext is given, it falls back to 'akka.actor.default-dispatcher.default-executor.fallback'.