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`. 
Because the default charset for SMTP is 7-bit ASCII. This version doesn't implement any extensions dealing with 8-bit and binary data, it is to be in the next one.