Add `create_adjustment` in ShippingMethod

Still need to figure out a place to trigger that method on the checkout
process.It should probably be done as a before_transition to payment.
Can't think of any other place right now given the current dependency we
have as everything is done via `order.update_attributes` and
`order.next`
Create shipment adjustments before transition to payment
Shipment cost attr to keep cost adjustment amount

While shipment adjustment_total would keep the sum of all shipment
adjustments amount
Remove Shipment update_adjustment callback

As I'm not sure yet if and when shipments adjustments might be
recalculated
Run order create_tax_charge after delivery step

Before transitioning to delivery step the shipments have no cost yet
because the no shipping rate was actually chosen at that point. Creating
taxes at that point may lead to errors as shipment `cost` is still nil
or to taxes with amount 0
Update order total after ship cost is created

Still need to refactor method calls and improve specs in OrderUpdater.
I'm not sure yet if we should count shipment and line item adjustment
towards or order.adjustment_total or keep them inside line item /
shipment itself scope. e.g. LineItem total would be:

  total = amount + adjustment_total