Handling successStatus with @SuccessStatus annotation

Providing @SuccessStatus annotation to override the default status=200 returned by a restx route.

See example below :

    @POST("/sessions")
    @SuccessStatus(HttpStatus.CREATED)
    public User authenticate(Credentials credentials) {
    ....
    }

See this [discussion](https://groups.google.com/forum/#!topic/restx/u9-KsCGPS3Q) for more context.

Note that I prefered to add a dedicated field in StdEntityRoute in order to be able to provide the information of success status in apidocs.
We should re-think this apidoc information if, someday, a "dynamic" status code is returned