Sets the status of the invoice.
        @param value:str

def __set_status(self, value):
        '''
        Sets the status of the invoice.
        @param value:str
        '''
        if value not in [INVOICE_DUE, INVOICE_PAID, INVOICE_CANCELED,
                         INVOICE_IRRECOVERABLE]:
            raise ValueError("Invalid invoice status")

        self.__status = value