Replace println with taoensso.timbre/trace

[eerohele]

I'm hesitant to create a PR that adds a dependency, but consider this a starting point.

If you'd prefer just to remove the println calls or use another logging library (such as clojure.tools.logging), just let me know.

[tatut]

That was my reasoning behind not using it, but timbre is my go-to logging library for Clojure as well, so it's fine :).

Thanks again. I'll cut a new release soon.