public.json: Split out product.images and category.images into signedImages

Instead of an array of URLs:

  [
    "http://a.com/image-1.jpg",
    "http://a.com/image-2.jpg",
    ...
  ]

use a list of signed Thumbor URLs [1].

  [
    {
      "480x480": "http://thumbor.a.com/<sig>/480x480/http://a.com/image-1.jpg",
      "800x800": "http://thumbor.a.com/<sig>/800x800/http://a.com/image-1.jpg"
    },
    {
      "480x480": "http://thumbor.a.com/<sig>/480x480/http://a.com/image-2.jpg",
      "800x800": "http://thumbor.a.com/<sig>/800x800/http://a.com/image-2.jpg"
    },
    ...
  ]

I'm using additionalProperties since we don't want to enumerate
possible keys [2,3].

[1]: https://github.com/thumbor/thumbor/wiki/Security
[2]: https://groups.google.com/forum/#!topic/swagger-swaggersocket/1ECmgh_ySmw
[3]: http://json-schema.org/latest/json-schema-validation.html#anchor64
     https://tools.ietf.org/html/draft-fge-json-schema-validation-00#section-5.4.4
     5.4.4. additionalProperties, properties and patternProperties