Use Jena's notion of graph containment instead of our strict equality

https://www.pivotaltracker.com/story/show/80907094
Yeah, I'm saying we actually fold up the `Graph` and return a non-Jena type. But this is, again, a question of just how much we're going to fight the Jena types percolating through our system. Maybe it's better to just go with this as it is, since I am going to come back at some point soon and factor equivalence out of this type anyway. That might be the time to worry more about it. {sigh}