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`. 
Hurk, how long do SMTP sessions last?  Do we start an SMTP session with a tcp connection and tear it down when we turn down the SMTP session, or do we set it up / tear it down with the request?