adding connection authentication to messageInfo & message

The `startMessage` event returns only `from`, `to` and `connectionId` at this moment. 

I must have access to the `authentication` data which are stored inside SMTP connection object. My mailin server acts differently based on `authentication.username` and `authentication.authenticated` values. 

This PR adds the `authentication` object to the `messageInfo` and `message` object. I was thinking of attaching the whole connection instead but I changed my mind - I think it's more optimized and aesthetic this way. I hope you will agree with this and merge. Thank you!