[Zope-PAS] challenge branch ready for review

[Zachery Bir]

I've got a working implementation of PAS on 
pre-1_0_3-zbir-challenge-branch that exercises:

  - the CookieAuthHelper plugin (very rudimentary, not as smart as
    CookieCrumbler)

  - the HTTPBasicAuthHelper

  - the new challenge machinery discussed here that limits players in
    a given challenge to plugins that support the same protocol

We've also got tests that exercise nested PAS instances, showing that
PASes that can't or don't participate in a challenge will delegate it
up the request chain and allow other PASes (or even the ZPublisher) to
challenge.

Please take a look and let me know what you think. I'd like to merge
this to the head and then start on the ID mangling (coming, Jens, I
promise ;^)).

Zac