iD.Difference represents the difference between two graphs.
It knows how to calculate the set of entities that were
created, modified, or deleted, and also contains the logic
for recursively extending a difference to the complete set
of entities that will require a redraw, taking into account
child and parent relationships.
Additionally, all history mutators now return a difference.
Disabling interactive draw undo for now. I know how to fix it
but it's a bit tricky. So undo will just drop you out to browse
mode for the time being.
Fixes#477.
Fixes#516.
John Firebaugh