Separate serializers for different endpoints

[TehMillhouse]

Currently, serializers just flat out don't work for anything but vanilla GET requests (see #413, #422). A pattern seen relatively often in the wild for REST interfaces is to provide only a listing of objects for the GET_MANY endpoint and only serializing all fields of an object for the GET_SINGLE endpoint.
i.e., something like GET /api/orders would return:

{
  "num_results": 2,
  "objects": [
    {
      "id": 1
    },
    {
      "id": 2
    }
  ],
  "page": 1,
  "total_pages": 1
}

While GET /api/orders/1 would return:

{
  "id": 1,
  "name": "Customer McBuy",
  "item_ids": [
    13,
    52,
    7
  ]
}

It would be very nice to be able to do this with Flask-ReSTless without taking a huge performance hit, however the current custom serializer support is very limited. My proposition is:
introduce a new keyword parameter to create_api called serializers which works just like pre/postprocessors: a dict which specifies for every version of every endpoint which serializer is to be used. (optionally deprecate the current serializer argument (as it's broken) or use it as fallback for endpoints not specified in serializers)

This would take care of #413 and #422 and incidentally scratch my own itch :)

[jfinkels]

The code has been completely rewritten to support the JSON API protocol, so I'm not sure if this issue still applies. Related to #475 as well.

[jfinkels]

This should be fixed by pull request #493.