v2017-10-23: Paginated Coupons

This version introduces pagination to the Coupons endpoint. Pagination is a standard feature for most Xola endpoints that return a collection of records. This change brings the Coupons endpoint in line with that standard.

Here are the changes to expect in the Coupons endpoint payload (GET /api/coupons).

{
  "data": [
    { // coupon 1 },
    { // coupon 2 }
  ],
  "paging": {
    "next": "/api/coupons?...",
    "total": 23
  }
}

[
  { // coupon 1 },
  { // coupon 2 }
]