Issues #92 and #93 - fixed

* extracted ILoggingConfig from LoggingConfig
* switched to interface usage everywhere
* modified LoggingModule logic to allow config source to be unregistered /
  config section to be missing; either case results in a default instance
  of LoggingConfig to be registered
* changed MoqRegistrationSource to only return registrations when creating
  new Mock-based ones; now returns an empty set in al other cases
* added new test assertions to verify the number of registrations before
  and after MoqRegistrationSource is used with a container instance