Update specification to OpenAPI version 3

[bougar]

At this moment, the version of the spec used in this project is old. It should be updated to OpenAPI version 3, in order to keep the project up to date and make use of new useful features. In fact, some of the features used on the spec provided by this project are using version 3 features which are not working, like nullable parameters.
On the other hand, the Rest API supports some features not collected by the swagger specification. I think it would be a good practice provide the same funcionality on the spec.