Basic inspecting_client and tests

Problem here is that any exceptions that are raised by the callbacks will be logged only by the tornado.app logger. Which in principle is fine, but does not work nicely with how logging is configured with the rest of the system. Basically, all of our components pass specific logger instances around to ensure the right stuff ends up on the central logger. Until we improve our system-wide logging behaviour, we probably need to support that kind of logging here, and also pass the logger instance to the lower level KATCP client.