Support validation of draft-04/hyper-schema

(Tests are part of the JSON-Schema-Test-Suite submodule; my PR hasn't been
merged upstream yet, though)

Given that I didn't actually need to change anything important to get the
validator to validate test schemas, I'm not entirely convinced that I've
done the right thing here -- or else the architecture of the validators is a
bit overengineered.
Merge pull request #125 from mpalmer/hyper-validation

Hyper validation