Add AWS3 authentication to a request.

        :type req: :class`boto.connection.HTTPRequest`
        :param req: The HTTPRequest object.

def add_auth(self, req, **kwargs):
        """
        Add AWS3 authentication to a request.

        :type req: :class`boto.connection.HTTPRequest`
        :param req: The HTTPRequest object.
        """
        # This could be a retry.  Make sure the previous
        # authorization header is removed first.
        if 'X-Amzn-Authorization' in req.headers:
            del req.headers['X-Amzn-Authorization']
        req.headers['X-Amz-Date'] = formatdate(usegmt=True)
        req.headers['X-Amz-Security-Token'] = self._provider.security_token
        string_to_sign, headers_to_sign = self.string_to_sign(req)
        boto.log.debug('StringToSign:\n%s' % string_to_sign)
        hash_value = sha256(string_to_sign).digest()
        b64_hmac = self.sign_string(hash_value)
        s = "AWS3 AWSAccessKeyId=%s," % self._provider.access_key
        s += "Algorithm=%s," % self.algorithm()
        s += "SignedHeaders=%s," % ';'.join(headers_to_sign)
        s += "Signature=%s" % b64_hmac
        req.headers['X-Amzn-Authorization'] = s