REP-819 Keystone V3 Filter

The mess that is Keystone v3! Comment on _everything_ so we can get this fixed up. =)
@adrianjgeorge I just checked and the filter director is indeed destructive when using put. Your point sounds valid to me, and I think appending the WWW-Authenticate header is the right call here. I did, however, take this logic from the Keystone v2 filter, so I wonder if there was a reason for doing it this way? Again, I defer to @RackerWilliams.