Added --mode=plain option to send_pgp_mime.

Added --mode=plain option to send_pgp_mime.

Also a few more tweaks to get things working.  I think be-handle-mail
is parsing the incoming messages correctly now, but I'm not getting
replies back for some reason.  Some of the adjustments:

  * Moved send_pgp_mime -> send_pgp_mime.py, otherwise Python doesn't
    recognize it as an importable module.
  * I use postfix now instead of msmtp, so send_pgp_mime.sendmail now
    points to postfix's sendmail-compatable frontend.
  * Added "--mode=plain" option to send_pgp_mime.py, so I can test
    my procmail rules and send_pgp_mime itself without worrying about
    be-handle-mail.
  * Fixed some typos in be-handle-mail.