Extracts logging, introduces Logger class

It's related to https://github.com/frosas/spotify-to-mp3/issues/40
I'd also like to have better logging here but didn't want to mess the current logic with random log-related code.

This delegates logging to a separate class so it's a base for future improvements.
I can do some work on logging with small steps when I have next spare 30 minutes, waiting for the feedback about this piece of code first.