Move factory up into the parent class

As I was porting one project from the old codebase to the new codebase, I realized that the factory would be able to be moved up into the parent model class, removing the need to add it to every model. 

The only time this couldn't happen is for people hosting on versions of php < 5.3, which is rare for most shared hosting these days. I added a note for this situation though to make the issue clear.