This is a module helper for developer who are looking
for integrate a subs payment system.
It alter subs type entities
to add further payment fields needed. Such as
currency code, amount, tax rate, quantity.
The idea is that each subs will used the
payment property of the subs type attached.
It provide an embeddable payment form to use
in a custom form for selling subs by providing
subs types as a "model" (amount, tax rate, and so on).
This module is integrate by the subs checkout module.
It integrate views by providing relationships
between subs and payments.