Use LSB on class instanciation to ease extending them

While working on https://github.com/gushphp/gush-gitlab-adapter/pull/4 I had some problem extending model classes as they were instancating themselves through static methods using hard coded class names.

I replaced as much as possible this by using LSB (with `static` keyword) so that, when extending models and using their static factory methods, we get instance of the right class.