added responseModel support

Include the entity defined in `http_codes` as a `responseModel`.

Example:

    desc 'This returns something or an error', {
      entity: Entities::Something,
      http_codes: [
        [200, "OK", Entities::Something],
        [403, "Refused to return something", Entities::Error]
      ]
    }
It's probably better not to include the `responseModel` attribute if a model is not specified, at least that would behave the same way the previous version did. Link: [my version](https://github.com/tim-vandecasteele/grape-swagger/commit/99b143051ae9cc349a3aab0643d19b312f36cc35#diff-3bdef0e908741c1b55e8e9660610e33dR385).