GetBalance and GetBalancePair

[Benjamin Cordes]

getBalanceImpl(CurrencyPair pair, Currency currency)

should be getBalanceImpl(Currency) only, and then a separate method to get two needed balances.

 @Override
    public ApiResponse getAvailableBalances(CurrencyPair pair) {
        return getBalanceImpl(pair, null);
    }

    @Override
    public ApiResponse getAvailableBalance(Currency currency) {
        return getBalanceImpl(null, currency);
    }