Pad value with bytes so it's a multiple of 16
        See: http://stackoverflow.com/questions/14179784/python-encrypting-with-pycrypto-aes
        :param data:
        :return data:

def _pad(self, data):
        """
        Pad value with bytes so it's a multiple of 16
        See: http://stackoverflow.com/questions/14179784/python-encrypting-with-pycrypto-aes
        :param data:
        :return data:
        """
        length = 16 - (len(data) % 16)
        data += chr(length)*length
        return data