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`. 
If we wanted to implement it the other way (constructing headers from fields, as there is access for fields and no access for headers - as far as I can remember, this is the case with javamail, for example), we would need to override it. Actually, it is much less cumbersome to make EmailBuilder an implementation of EmailMessage this way. 
Here collecting fields from headers is just more general, I think.