List of payment modules of a shop.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
name	TEXT		Payment module's name. It assumed to be same to directory's name under the $CASSIOPEIA_HOME/module/payment.
index	INTEGER	NOT NULL	Ordering of payments. 0-origin.

CREATE TABLE payment_module(
shop    TEXT,
name    TEXT,
index   INTEGER  NOT NULL,
PRIMARY KEY (shop, name)
);