Run json-schema.org's common test suite

Implementation of #113.

Straightforward generation of test cases based on the contents of the JSON-Schema-Test-Suite repo.  I decided to go wild and add it as a git submodule; if that's not cool, then some other method of dealing with the problem can be devised.  At least I hid all the insanit`^W`magic behind the rake task, so nobody needs to deal with it by hand.