Update order.item_total before activating promotions

Also need to pass the order object already loaded in memory to
PromotionItemHandler because that's the object that contains the
updated, but not persisted yet, item_total
Include order.adjustments to set order.adjustment_total

The store should still be able to apply order level adjustments over
order total
Include CreateItemAdjustment action in engine initialization

So CreateItemAdjustment has only the FlatRate calculator available. I
think others will have to be created to play nice with a line_item
object