Review logging code and levels

[dantswain]

This is inspired by the conversation in #157. There are some places where the log level might need to be tweaked or the logging could be removed completely. We may want to consider allowing the log level for some events to be configurable and/or using the Logger.info etc family of functions to allow the :compile_time_purge_level setting to take effect.

I think we could take some cues from how Ecto handles logging: https://github.com/elixir-ecto/ecto/blob/v2.1.2/lib/ecto/log_entry.ex