[GRAPHS] Use Cases wiki page

All,
I've started a wiki page for collecting the use cases that were assigned as
action items at the last meeting:

http://www.w3.org/2011/rdf-wg/wiki/TF-Graphs-UC

Some if this stuff is stupid-simple, but I figure it's better to err on the
side of inclusiveness.  Add your own, and feel free to re-organize as you
see fit.  I'm thinking we should just dump everything we have in one place
and then sort it out once everybody has added their own use cases.

Regards,
Alex