Changes

tsk21-40 Adds Story and User DTO
Adds the DTOs of the Story and User entities.
Made by Alejandro and Rubén.

tsk21-40 Modifies the REST methods of the Story class
Modifies the REST methods of the Story class to use the created DTOs. 
Also adds Transactional annotation to the necessary methods, to maintain 
an open transaction in this layer.
Made by Alejandro and Rubén.

tsk21-40 Modifies REST tests of the Story class
Modifies the REST layer tests to adapt them to the use of DTOs.
Made by Alejandro and Rubén.