[Email-SIG] Message threading code

[Matthew Dixon Cowles]

I've uploaded my implementation of Jamie Zawinski's message threading
algorithm to:

http://www.mondoinfo.com/messagethread.py

It took some significant surgery to extract it from its previous
home, but I think that it's still correct. If anyone's interested in
it, you're welcome to it as it stands. If Barry thinks it's a good
start for something in the email module, I'll be happy to clean it up
according to the python style guide and assign copyright to the PSF.

Regards,
Matt