Allow `belongs_to` with `primary_key` option

Created by: jcmorrow

Resolves #777 (closed).

Changes:

• Allow permitted_attribute to receive options, which can include primary_key. Use primary_key if specified, otherwise use id.
• Change orders primary key to code and payments foreign key to order_code.
• Remove created_at and updated_at from Payment's ATTRIBUTE_TYPES because the payments table doesn't actually have those columns.
• Add edit route for payments.