More and finer grained data

This pull request allows a few things:

* pulls in message headers from Mailgun
* makes smtp message id available
* makes in_reply_to id available
* stores cc information
* stores bcc information
* methods to determine if an email is a bounced response / vacation responder
* reformats email addresses when Outlook badly rewrites them (and keeps the original available)
* allows fetching body_text and body_html separately
* allows fetching the signature & replies block
* a method that returns all email addresses included in an email (to, cc, bcc, emails in body)

We're using this functionality in production, thought it might do better being part of this gem. Added specs to cover the relevant functionality.