Introduce logging module for ability to control output

This adds a basic logging module so that the output of this module can be controlled by its user. By default the log level is set to `error`, but when `NODE_ENV` is `development` it is set to `info`.

Additionally, the user may override without using `NODE_ENV` by setting `DISCOVERY_LOG_LEVEL` to an approved winston console transport log level.

Very helpful when debugging an application which consumes `node-sonos-discovery` so that the internal logging doesn't drown out application logging.