ACTION-200 (text for ISSUE-84)

I believe this got assigned to me because the group was unhappy with the
original draft at
http://lists.w3.org/Archives/Public/public-tracking/2012Feb/0609.html that
Tom proposed as an update, but I'll admit my memory is a bit fuzzy.

Rather than going down and specifying the exact API / IDL, I'd like to try
to first specify use cases and see if we agree on that..

A frame should be able to discover:

a) whether it received DNT:0 or DNT:1
b) whether a user has approved a site-wide ("this-site/*") exception for
the current frame

If we go further, we get into fingerprinting risks, and it also depends on
whether we're supporting finer grained exceptions...