Add scynapse-akka module

`scynapse-akka` implements an Axon event bus to Akka bridge as Akka extension.
I did not find a way to do that because Akka extension is created internally using `ExtensionId.createExtension(system: ExtendedActorSystem)` (http://doc.akka.io/api/akka/2.3.6/#akka.actor.ExtensionId) that takes only ActorSystem as an argument. And I think my resulting approach is a bit more flexible because (for some reason) you may use multiple event buses in the project.