Add support for $ref in responses

[marcogiovinazzi]

This PR adds support for reusable responses ($ref) as described in:

• https://swagger.io/docs/specification/describing-responses/
• https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#responses-object

Unit tests added to cover the new functionality. Bye and thanks for this great lib!