Add a SMTP adapter

[lemunozm]

Add an adapter to send mails easily through message-io as if they were messages.

Things into consideration:

• The messages should be sent in text. What happens if somebody sends binary data?
• At connection phase, how to pass the remittent and server info? Encoded by RemoteAddr::Str or by Adapter with configuration properties by connection/listening #54
• How to receive messages (based on mio poll)? The internal Waker should be exposed to the adapter API?

[lemunozm]

Out of scope at this moment