Load messages previously saved to S3.

def load_from_s3(self, bucket, prefix=None):
        """
        Load messages previously saved to S3.
        """
        n = 0
        if prefix:
            prefix = '%s/' % prefix
        else:
            prefix = '%s/' % self.id[1:]
        rs = bucket.list(prefix=prefix)
        for key in rs:
            n += 1
            m = self.new_message(key.get_contents_as_string())
            self.write(m)
        return n