fix authorizations

Pull request for the issue https://github.com/wordnik/swagger-js/issues/127

* Calls to the authorizations are only made when requesting an operation, and not when requesting the resources list, or any resource (according to the RFC)
* Operations without any `authorizations` field inherits from the `authorizations` field of its resource
* Empty object `{}` as `authorizations` field means no authorization required
* All the authorizations of the `authorizations` field are required