Update to Abstract Identifier Architecture page

Per my action item from the XRI TC telecon this week, I posted an update to
our Abstract Identifier Architecture page:

	http://wiki.oasis-open.org/xri/AbstractIdentifierArchitecture

Key changes:

1) As pointed out by Dan Connolly in his post to the W3C TAG public mailing
list...

	http://lists.w3.org/Archives/Public/www-tag/2008Sep/0083.html

...we had not stated an implicit assumption that our definitions pertained
in the context of a specific communications protocol. I fixed this.

2) I added a graph diagram to help make it very explicit how abstract
identifiers, concrete identifiers, resource descriptors, and resource
representations are related.

3) I added tables showing how this conceptual architecture applies to DNS,
DOI, and XRI.

4) I added one key conclusion these definitions lead to, which underscores
why doing metadata discovery via the same protocol as resource
representation retreival is such as challenge.

Comments/suggestions/improvements welcome. 

=Drummond