Added support for adding meta objects from serializers

[jerel]

Solves #154.

By declaring a meta_fields property on the serializer's Meta class meta data can be added to the:

• meta key of each item in a list serializer (a blog entry)
• meta key of a single data object (a blog)

By declaring a get_root_meta function on a serializer data can be added to the:

• top level meta object

{
  "data": [
    {
      "type": "entry",
      "id": "1",
      "attributes": {
        "title": "Test",
      },
      // set by EntrySerializer `meta_fields`
      "meta": {
        "bodyFormat": "text"
      }
    }
  ],
  // set by the `get_root_meta` function on EntrySerializer
  "meta": {
    "bar": "Baz"
  },
}

[jsenecal]

Nice work here @jerel :)

[jerel]

This is ready for review and feedback or merge

[Elawphant]

It seems, this does not work as expected.
I have extended rest_framework_json_api.serializers.ModelSerializer, added rest_framework_json_api.serializers.SerializerMethodField and get_my_field method to the serializer, and added my_field to Meta's meta_fields. However, instead of meta, my_field has appeared inside data.
get_root_meta on the serializer does nothing, either.

[auvipy]

I think you should open a PR with a failing test