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`. 
what I'm saying is that there's no point in saving signal off into a val.  this code is equivalent to:

```scala
decodeReply(rp, p)
p onFailure {
 ...
}
Future.Done
```

This also makes it more clear that decodeReply is mutating, and doesn't return anything interesting.  Once we're no longer using the return value, it's much more natural to make decodeReply return Unit, which is also semantically more correct since it's not actually asynchronous.