filtering header draft rev 38

I've edited the filterig header draft some more. You can download
a current copy at http://millenix.zemos.net/asrg-filtering/

Changes are a new Examples section where I've sketched what the
prototype filtering header might look like in practice. This way we
can see if it's awkward, and change the look of it if needed. 

It would be a good idea for people to sketch what the new standard
header would look like for many of the existing filters. We've got a
description of current header fields for some filters in this mailing list
archive already.

I've also fleshed out the authentication section some more.

-- 
Laird Breyer.