Turn LoggingAdapter to an interface and add new baseclass

Rename LoggingAdapter to LoggingAdapterBase.
Add new interface LoggingAdapter.
Make Logging.GetLogger an extension method on IActorContext

The preferred way of getting a logger inside an actor is:
private readonly LoggingAdapter _log = Context.GetLogger();
Merge pull request #454 from HCanber/ilogger

Replace LogAdapter with ILogger