NodeDiff:
 * status [string; one of "same", "added", "removed", "changed"]

EdgeDiff:
 * status [string; one of "same", "added", "removed"]


<DiffSet>
 <Nodes>
  <NodeDiff status=""></NodeDiff>
  ...
 </Nodes>
 <Edges>
  <EdgeDiff status=""></EdgeDiff>
  ...
 </Edges>
</DiffSet>

Add spec for diff set file format.
Add spec for diff set file format.
