Register a namespace.

        :param namespace: Namespace tag.
        :type namespace: str
        :param module: must be a string or a module object to register.
        :type module: str

def register_namespace(self, namespace, module):
        """
        Register a namespace.

        :param namespace: Namespace tag.
        :type namespace: str
        :param module: must be a string or a module object to register.
        :type module: str
        """
        if namespace in self._namespaces:
            raise AlreadyRegisteredError("Namespace '{0}' is already registered on loader.".format(namespace))

        self._namespaces[namespace] = module