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`. 
should be def toString(): String = cmd.  since cmd and toString are the same, maybe you should just make rename the argument to Request toString?  or do you think that would be confusing.