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`. 
Also, is there a reason you're explicitly returning `Future[Unit]` here? The inferred return type of `res` would be `Future[Reply]`, and the `Future[Unit]` return type only works because of Scala's value discarding, which is frequently very confusing.