Significant update to jb2bz.py. (#58)

Significant update to jb2bz.py. (#58)

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1427626
Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1427638
Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1427664

Convert jb2bz to use "email" module rather than rfc822, multifile,
mimetools, and StringIO for easier manipulation of the mailbox objects.

This fixed (among other things) 1427626 as the Subject of the email is
now obtained correctly.

Fix the date formatting throughout to always use UTC, which is what
Bugzilla expects for the dates.  This fixed 1427638.

Fix attachment processing, which was using multifile rather than walking
the parts of the email object.  This fixed part of 1427664

Fix the fact that the process_reply_file function never checked for
attachments in any followups.  This fixed part of 1427664

Fix attachment processor to ignore various signatures and message/rfc822
multipart messages.  For the latter, it sets "filename=" values to files
that don't actually exist, which caused attachment processing to bomb.