Demonstration of using json-schema for basic validation

Built off of work from @ottonomy. Defines the badgeClass and badgeAssertion in json-schema. Basic test suite demonstrates using jjv to validate against the schemas.

If all this looks right, I'd like to take a stab at moving [openbadges-specification](http://github.com/mozilla/openbadges-specification) to a module that exports these schemas, then refactor [openbadges-validator](http://github.com/mozilla/openbadges-validator) to use these schema and jjv for the first level of validation.