Break dependency on NLog

[Tim Long]

Its not great to have dependencies on too many third party libraries. We can't help but depend on System.Reactive, as that's the whole point, but we probably shouldn't depend on NLog.

Suggested strategy

1. Introduce an ILoggingService interface.
2. Inject the logging service as a constructor parameter were needed.
3. Make a degenerate implementation that does nothing, and make that the default.
4. Remove dependency on NLog.
5. Create a separate project/assembly/nuget package and Implement the interface as NLogLoggingService.

This may be a breaking change unless we can overload all the required constructors.