Apply coupons take two aka "The Coupon Applicator"

With 387650b a few weeks ago, I added a CheckoutsController to the spree API that is further explained at http://api.spreecommerce.com/v1/order/checkouts/

This is take two on this after receiving feedback last week. @radar, hoping this is more what we are looking for now that split_core is in master. I can expand on the CouponApplicator specs if that makes sense.

As always, feedback is welcome!