remove reference to shipment.adjustment

shipment.adjustment no longer exists so we can kill this. also wasn't
tested, so no need to change test cases.

My understanding is with the adjustment refactoring we don't need to
deal with the open/close anymore anyway.
Better errors on coupon code failing

Assuming everything that wouldn't activate properly was "already
applied" doesn't work since anything that fails to activate but wasn't
"already applied" shows up to an end-user as "already applied". By
explicitly checking for coupons already applied we can put a more
generic error if activate fails.
Encapsulate handling of payment preconditions

This would allow us to create methods that would verify the integrity of
a payment but allow us to do custom things with it.
Add utility methods for explicit authorization and capture.

There are times in custom applications that we explicitly want to
authorize or capture regardless of the payment_methods auto capture
preference or the store configuration. This lets us do that while
maintaining that the regular preconditions for payments are met.

Conflicts:
	core/CHANGELOG.md
	core/spec/models/spree/payment_spec.rb
Refactor process_payments!

As with the payment changes, this allows us to call custom methods on
payments for processing.

Conflicts:
	core/app/models/spree/order.rb
Add authorize_payments! and capture_payments! to orders.

Allows us to specifically authorize or capture payments on an order.

Conflicts:
	core/spec/models/spree/order/payment_spec.rb
Merge pull request #6 from freerunningtech/2-2-idempotent_processing

Refactor payment/authorization processing
Advance the order when approved

After approving an order, the action should try to advance the order.