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`. 
`isEmpty()` here is the method of `MailingAddress`, not `String`, but I can add `nonEmpty()` too, if it seems necessary.