LibroController implemented GestorLibro Implemented
Singleton pattern added to Gestors classes. It´s necesary to use other
Gestors as a Services
PrestamoController and GestorPrestamo implemented.
Fixed some bugs
NOTE: In the 2 previous commits, when I said LibroController and
GestorLibro I mean UsuarioController and GestorUsuario

App is running properly
Tests have been passed successfully
Bug reparado