Adding option to include invoiceNumber in PayPalPayment.

I need set the invoiceNumber in my project to associate Paypal payments with my own payment records that i need to create before launch the singlePaymentUI.
it doesn't correspond to the implementation in .java and .m classes. Anyone will try to use it that way will get a crash. Also, custom and invoiceNumber are two different things. The comment is misleading since PayPalPaymentDetails is optional as well and also property of the payment.