- Add CurrencyHandler with NewCurrencyHandler constructor
- Add ConvertCurrency with multi-currency support (USD, EUR, GBP, JPY, CAD, AUD, CHF)
- Add GetExchangeRate, GetExchangeRateHistory with 30-day historical data, and RefreshExchangeRates endpoints
- Add GetSupportedCurrencies returning code, name, and symbol for 10 currencies
- Add ProcessBilling with charge/credit/refund types and transaction creation