Parse all email type headers

[Slamdunk]

• Add tests for Implement "undisclosed recipients" addresses #86
• Parse Bcc, Reply-To, Sender and Return-Path as EmailAddress
• Implement (some of) the Date found in https://gist.github.com/mikesart/b33762363153e2b8c7c7
• Move to DateTimeImmutable
• EmailAddress remove __toString(): http://phpsadness.com/sad/14

BC Breaks

1. Removed Ddeboer\Imap\Message\EmailAddress::__toString