Use the Ruby agent's logging, plus small logging cleanup.

Many customers strongly prefer having none of New Relic's logging going to stdout/stderr. However, in the early stages of a project, it can be convenient to log to stdout. Thus, while I think we will *eventually* want to merge this in, it's unclear to me if we want to merge it in *now*.

But when we do want to merge it in, here it is. :-)