Schema $ref does not handle URIs

[InfoSec812]

When using externally specified schemas (e.g. From Schema.org), the $ref should be able to be an external URI which slush-vertx should automatically incorporate.

Example snippet:

      responses:
        '202':
          description: User is authorized, image ID exists, and tag is formatted correctly
          content:
            application/json:
              schema:
                $ref: 'http://schema.org.apis.apievangelist.com/api-commons/image-object/openapi-spec.yaml#/definitions/ImageObject'

[slinkydeveloper]

@InfoSec812 I'm moving all code generation on new pmlopes project https://github.com/pmlopes/vertx-starter on this new project the external refs should work