Return the ::base:: to ::target:: exchange rate.

def get_exchange_rate(self, base, target, raise_errors=True):
        """Return the ::base:: to ::target:: exchange rate."""
        assert base and target

        base, target = base.lower(), target.lower()

        r = self.session.get(API_SIMPLE_TICKER.format(base, target))
        if r.status_code != requests.codes.ok:
            if not raise_errors:
                return None
            raise CryptonatorException(
                ("An error occurred while getting requested exchange rate "
                 "({} from Cryptonator).").format(r.status_code)
            )

        j = r.json()
        if not j['success'] or j['error']:
            if not raise_errors:
                return None
            raise CryptonatorException(
                ("An error occurred while getting requested exchange rate ({}, {})"
                 "('{}').").format(base, target, j['error'])
            )

        return float(j['ticker']['price'])