Establish the interface for showing diffs of a law

[waldoj]

After we can view old versions of a law (#362), then we'll want to be able to compare changes. Look to WordPress 3.6's revisions viewer. They've got a great little widget that I think will work nicely.

[krusynth]

Propose moving Milestone to Future.

[waldoj]

Yeah, I think just storing and displaying old versions is enough for v1.0.