SMTP quite working

Better mailboxes, better EHLO, MAIL, QUIT, RCPT and RSET, added VRFY,
added the file (doesn't work yet though) for STARTTLS, changed a bit
the session and the config.

Now using the rfc2822 package (github.com/trapped/rfc2822) to parse
and edit standard emails (multipart is complex). It's very handy, and
also a fork from eevans (github.com/eevans/rfc2822).