def new_notebook(self):
        """Create a new notebook and return its notebook_id."""
        path, name = self.increment_filename('Untitled')
        notebook_id = self.new_notebook_id(name)
        metadata = current.new_metadata(name=name)
        nb = current.new_notebook(metadata=metadata)
        with open(path,'w') as f:
            current.write(nb, f, u'json')
        return notebook_id
Create a new notebook and return its notebook_id.