def payments_for_address(self, address):
        "return an array of (TX ids, net_payment)"
        URL = self.api_domain + ("/address/%s?format=json" % address)
        d = urlopen(URL).read()
        json_response = json.loads(d.decode("utf8"))
        response = []
        for tx in json_response.get("txs", []):
            total_out = 0
            for tx_out in tx.get("out", []):
                if tx_out.get("addr") == address:
                    total_out += tx_out.get("value", 0)
            if total_out > 0:
                response.append((tx.get("hash"), total_out))
        return response
return an array of (TX ids, net_payment)