Let the application specify custom MDC values

Hi guys,

With some modifications to LoggingAdapter, BusLogging and Slf4jLogger I was able to let the application specify custom MDC values when logging any message.
This new feature is hard to use as it is, but combined together with an Akka pattern I'm developing ( https://github.com/ktonga/akka-contextual-actors ) the experience of being using a transparent MDC solution gets a little bit more real.

Let me know if you need some further modifications!
Thanks.