Download a file to local disk.

        :param dest: a path or a ``file`` object

def download_file(self, file_id, dest):
        """
        Download a file to local disk.

        :param dest: a path or a ``file`` object
        """
        f = self.getFile(file_id)
        try:
            d = dest if _isfile(dest) else open(dest, 'wb')

            r = api.download((self._token, f['file_path']), preload_content=False)

            while 1:
                data = r.read(self._file_chunk_size)
                if not data:
                    break
                d.write(data)
        finally:
            if not _isfile(dest) and 'd' in locals():
                d.close()

            if 'r' in locals():
                r.release_conn()