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.
Hey I think this line breaks the current api. Would it be possible to rewrite it like this:
```javascript
_this.emit('message', parsedEmail, _.merge({}, connection, connData), rawEmail);
```
So that it does not introduced api breaking changes?