Adding support for other methods

Now I added support for the following API methods (these are only
wrappers):
Express Checkout
SetExpressCheckout (NVP) (SOAP)
GetExpressCheckoutDetails (NVP) (SOAP)
DoExpressCheckoutPayment (NVP) (SOAP)
Callback (NVP)

Authorization and Capture
DoAuthorization (NVP) (SOAP)
DoCapture (NVP) (SOAP)
DoReauthorization (NVP) (SOAP)
DoVoid (NVP) (SOAP)
UpdateAuthorization (NVP) (SOAP)

Recurring Payments / Reference Transactions
BAUpdate (NVP) (SOAP)
BillOutstandingAmount (NVP) (SOAP)
CreateBillingAgreement (NVP) (SOAP)
CreateRecurringPaymentsProfile (NVP) (SOAP)
DoReferenceTransaction (NVP) (SOAP)
GetBillingAgreementCustomerDetails (NVP) (SOAP) — Deprecated after
version 54.0
GetRecurringPaymentsProfileDetails (NVP) (SOAP)
ManageRecurringPaymentsProfileStatus (NVP) (SOAP)
SetCustomerBillingAgreement (NVP) (SOAP) — Deprecated after version 54.0
UpdateRecurringPaymentsProfile (NVP) (SOAP)

Refunds
RefundTransaction (NVP) (SOAP)