Implement digestor class.
Add digestor to form builder.
Implement GateKeeper to authenticate form.

Including digests.
Add digestor stores.
Add an engine to render form expired view.
Add a bad digest grace period.

This is useful for minor changes that you don't want to invalidate a
form that a user is currently filling out.
Add change log entry.
Update README.
Merge branch 'digestor'