compare receipt date of most recent unread mail to determine if a new
message has arrived since last check. this prevents spurious new-mail
notifications (when mails are read or deleted).

also, break down unread counts by account in tooltip and improve growl
formatting a bit.
Made Growl notifications a little saner.

1. One Growl notification per new email.
2. But only show notifications if a notification for this email has
   not been shown before since the app started up.