Fix for issue #39

Added a new constructor to the `Hibernate4Module` where you can pass your Hibernate `Mapping` object. Usage example:

        // Create Hibernate configuration & get build mapping.
        final Configuration configuration = new Configuration().configure("hibernate.cfg.xml");
        final Mapping mapping = configuration.buildMapping();

        // Create Hibernate4Module with Hibernate mapping.
        final Hibernate4Module hibernate4Module = new Hibernate4Module(mapping);

        // Register Hibernate4Module with ObjectMapper.
        final ObjectMapper mapper = new ObjectMapper(new JsonFactory());
        mapper.registerModule(hibernate4Module);