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`. 
It'd be better to keep this example out of the project proper (and definitely out of the root of the `com.twitter.finagle` package). The best place at the moment would be `finagle-example`, but I'll follow up over email about other ideas.