Begun work on #6 support for Python3

Most of the code has been tested on python2/3 and appears to be working correctly for both. It's almost finished but I haven't tested the example/mail_relay or secure_smtpd/proxy_server, all the other examples runs fine.

I'm not so experienced with git so I decided the easiest way to merge in my branch is to (on github) create a new branch on this repo and do a PR and merge it. Hopefully this won't mess up any repos.