Organize .gitignore, qualify toplevel ignores, add .ruby-version and /bin ignores
Fix that Outlook and Windows Phone treat all our MIME parts as inline attachments since they include unused, unreferenced Content-IDs.

Closes #389
Expose the SMTP envelope From and To addresses and allow them to be overridden.

Envelope From address defaults to return_path || sender || from_addrs.first.
Envelope To address defaults to destinations (to + cc + bcc).

Updates all delivery methods to rely on the SMTP envelope.

References #421 and rails/rails#5985