renaming `smtp` variable to `_smtp`, simplesmtp events proxy, uniforming emitters design, upgrading startMessage & message events

It took me almost a day to figure the right concept for this :). I tried to stay close to your initial concept and naming but I had to make some minor changes as well. All changes should be safe because I check the simplesmtp code.
Mmm.. You're right this makes sense. If we follow semver, we might bump to version 1.0.0 then.