Sets the method to use.
        @param value: str

def __set_method(self, value):
        '''
        Sets the method to use.
        @param value: str
        '''
        if value not in [DELIVERY_METHOD_EMAIL, DELIVERY_METHOD_SMS,
                         DELIVERY_METHOD_SNAILMAIL]:
            raise ValueError("Invalid deliveries method '%s'" % value)

        self.__method = value