GSoC: finagle-smtp - initial

This is initial version of SMTP client for Finagle, which I am working on during GSoC'14.
For now it supports sending independent commands to SMTP server and sending a simple text email to one ore more addresses (without cc, bcc and reply-to). Usage is described in `README.md`. 
If you mean that email addresses should be checked to look like user@domain.com, this is done when constructing MailingAddress'es from them. I can add this type of requirement here too.