Make auto_capture? a per payment configuration

Allows of payment methods to have auto_capture on/off per gateway. Right now, it is a global config Spree::Config[:auto_capture] that can be set in Spree. To make it per-payment method, the recommended approach is to provide an auto_capture? method on the object you want to change.

http://guides.spreecommerce.com/developer/payments.html

This change will make it less of a developer change and more of an Admin change. When set to nil, the payment method will continue to respect the global config. But if it is set to true/false, then auto_capture will be per payment method.