Reworked validation logic to properly distinguish 'must_revalidate' from 'can_revalidate_locally', should fix #62
Reworked validation tests and added re-revalidation test
Added support for tables of pretransition actions to be executed in order
Rerevalidation. This allows us to recompare a fetched response against the original client validators, and 304 where possible. This should all be more spec compliant now. Closes #62