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`. 
in general, we use 0-arity methods without parentheses if it acts like a field.  `build()` doesn't act anything like a field, so let's use parens.