public.json: Factor out 'price' into an object

Collect the dollar value, discount, and per-pound (or per-unit) status
in their own structure.  This keeps the root product namespace
organized and forms the core of a future price API, while allowing
folks using the product API to distinguish between per-pound and
per-unit prices.
Merge branch 'wtk/product'

* wtk/product:
  public.json: Factor out 'price' into an object
  public.json: Add /products and /product/{code}