def account_balance(self, base="btc", quote="usd"):
        """
        Returns dictionary::

            {u'btc_reserved': u'0',
             u'fee': u'0.5000',
             u'btc_available': u'2.30856098',
             u'usd_reserved': u'0',
             u'btc_balance': u'2.30856098',
             u'usd_balance': u'114.64',
             u'usd_available': u'114.64',
             ---If base and quote were specified:
             u'fee': u'',
             ---If base and quote were not specified:
             u'btcusd_fee': u'0.25',
             u'btceur_fee': u'0.25',
             u'eurusd_fee': u'0.20',
             }
            There could be reasons to set base and quote to None (or False),
            because the result then will contain the fees for all currency pairs
            For backwards compatibility this can not be the default however.
        """
        url = self._construct_url("balance/", base, quote)
        return self._post(url, return_json=True, version=2)