Updates to XDI Message Pattern wiki page

FYI (especially to implementers): Based on today's telecon, I took care of the long overdue action item of updating our XDI Message Pattern page on the wiki...

   http://wiki.oasis-open.org/xdi/XdiMessagePatterns

...to include the link contract reference statement that we agreed upon in June but had never fully finalized. 

This is important because the OpenXDI Project had already started coding something that was very similar but included an extra $do context in the template, which when we finally got down to documenting it we realized was: a) not needed, and b) actually confusing semantically, because it made it look like the link contract reference was part of the link contract, when it's actually just part of the message.

When editing the wiki page, I also made two other small corrections:

The $() predicates were changed to $is() per our decision in June.

I noticed that the templates had ($id} as a variable when it should have been just {id}.

I believe everything is now current.

=Drummond