Re-enable nested OrderItems.
Merge pull request #90 from gregwym/api

Re-enable nested OrderItems.
Rename to ChildrenListModelMixin and inherit ListModelMixin.
Use correct parent name for model name longer than one word.
Merge pull request #91 from gregwym/api

Use correct parent name for model name longer than one word.
Set coupon.used = False if payment has been cancelled.
Merge pull request #92 from gregwym/reenable-coupon

Set coupon.used = False if payment has been cancelled.