Add custom logger functionality for CacheLevels

[vittoriom]

Basic idea is that every CacheLevel has its own logger reference. Logger is a protocol with a log function.

Breakdown would be:

public enum LogLevel {
    case Debug, Info, Warning, Error, Fatal, None  
}

public protocol Logger {
    func log(message: String, level: LogLevel)
}

and then

public protocol CacheLevel {
   [...]

   func setLogger(logger: Logger)
}

Then one can call setLogger on whatever CacheLevel (for example the top-most, that will set the logger to all its managed cache levels, or whichever you need to).

[vittoriom]

Won't fix because it's IMHO too complex and the reward is not worth the effort