Add to-many relationship format as an array of link objects

Initially [proposed][ac] by @mikelinington in json-api/json-api#183, this expands the to-many relationship format to accomodate an array of `link` objects.

[ac]: https://github.com/json-api/json-api/issues/183#issuecomment-33840493

```json
{
  "posts": [
    {
      "id": "1",
      "title": "One Type Purr Author",
      "links": {
        "authors": [
          {
            "href": "http://example.com/people/9",
            "id": "9",
            "type": "people"
          },
          {
            "href": "http://example.com/cats/1",
            "id": "1",
            "type": "cats"
          }
        ]
      }
    }
  ]
}
```