Add custom constructors to multiple loaders

When someone writes a subclass of the YAMLObject class, the constructors
will now be added to all 3 (non-safe) loaders.

Furthermore, we support the class variable `yaml_loader` being a list,
offering more control of which loaders are affected.

To support safe_load in your custom class you could add this:

    yaml_loader = yaml.SafeLoader

    yaml_loader = yaml.YAMLObject.yaml_loader
    yaml_loader.append(yaml.SafeLoader)