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`. 
1. Generally the "getFoo" names aren't used in Scala due to the Uniform Access Principal. You can simply implement `def foo` (and `def foo_=` for a setter).
2. It seems that general header access of the form `def headers: Seq[(String, String)]` is desirable; and then things like `from` can simply be implemented on top of it like:
```
def from: Seq[MailingAddress] = headers collect { case ("From", addr) => MailingAddress(addr) }
```