= util: add small and simple cache for creating settings from an ActorSystem
! util: remove SprayActorLogging and UtilSettings, simplify LoggingContext, fixes #421

Since not all SLF4J implementations (most importantly logback) can properly manage a potentially unbounded number of loggers we cannot use actor paths as logger names. The latter would have been nice since, by converting actor paths to dotted notation, we'd be able to use hierarchical configuration as well as logbacks abbreviation feature. However, since we can't use actor paths as logger names the feature of "dottifying" actor paths is not useful.

The `SprayActorLogging` and `UtilSettings` abstractions therefore don't serve any purpose anymore.
Please replace all your usage of `spray.util.SprayActorLogging` with `akka.actor.ActorLogging`, which provides the exact same API.
Merge pull request #452 from spray/ticket/421

Fix #421: SprayActorLogging and LoggingContext creating memory leak