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`. 
thinking about this more, I think this is incorrect.  we don't want to close the connection when we close this service, we just want to relinquish the connection so that it can be used elsewhere.  this should definitely be moved into the dispatcher.