Change license to EUPL
First compilation

Currently not conforming to RFC5321. Commands accepted:
- HELO
- EHLO
- MAIL FROM:<bob@example.com> (No name)
- RCPT TO:<alice@example.com> (No name)
- DATA
- QUIT

It handles each connection on a separate goroutine
The software is able to save incoming mail into a Maildir tree.
Only incoming mail at the moment.