Register any extensions under the given namespace.

def _register_extensions(self, namespace):
        """Register any extensions under the given namespace."""

        # Register any extension classes for this class.
        extmanager = ExtensionManager(
            'extensions.classes.{}'.format(namespace),
            propagate_map_exceptions=True
        )

        if extmanager.extensions:
            extmanager.map(util.register_extension_class, base=self)

        # Register any extension methods for this class.
        extmanager = ExtensionManager(
            'extensions.methods.{}'.format(namespace),
            propagate_map_exceptions=True
        )
        if extmanager.extensions:
            extmanager.map(util.register_extension_method, base=self)