* All default values removed from code and loaded from akka-actor-reference.conf, located in src/main/resources (included in jar)
* Default test configuration included in AkkaSpec instead of using akka.test.conf, avoids problems when running test (in IDE) and forgetting to use -Dakka.mode=test.
* System.properties used first, if availble
* Next step will be to split akka-actor-reference.conf in separate -reference for each module
- create ActorSystemImpl trait to make ActorSystem fully abstract
- add Java API for constructing (ActorSystem.create(...))
- only go through factory methods because .start() has become necessary
- rename all user-facing occurrences of “app” to “system” (Actor trait
and TestKit/AkkaSpec)
- pass ActorSystemImpl to ActorRefs upon creation, which means that
actorOf() and friends need such an argument, which must be provided to
the ActorRefProvider by the ActorRefFactory implementation
