Use logger instead of `p` the responses

[bararchy]

It seems there are prints to STDOUT hardcoded in the SMTP client class, do you mind allowing the user to pass in a Logger class and use it ? You could default to using Logger.new(STDOUT) or something

[ray-delossantos]

@bararchy, that's correct, right now i don't have the time for making this changes, but PR are welcomed :).