Modified json.js to allow non-capturing validation
Modified json.js to allow non-capturing validation

* json plugin can now accept a string (JSONPath) or an
array of JSONPath strings and each one will be tested for
existence against the response.
* If json is evaluated against a JSON containing string
but the response was not coded as application/json, it will
implicitly JSON.parse the response first.
* Added defaults.yml to test defaults functionality

pmarkert/hyperpotamus,pmarkert/hyperpotamus