Separate related resources in a top-level `linked` object.

It's important to separate the primary resource from related resources because:

* It allows the client to easily identify the primary resource among related resources of the same type. This is especially important when creating a resource without prior knowledge of its server-assigned id.
* It clarifies issues like pagination that are specific to the primary resource.

This can be achieved by nesting related resources in a `linked` object. For instance:

```javascript
{
  "posts": [{
    "id": "1",
    "name": "Rails is Omakase",
    "links": {
      "author": "9",
      "related": ["2"]
    }
  }],
  "linked": {
    "posts": [{
      "id": "2",
      "name": "The Parley Letter",
      "links": {
        "author": "9",
        "related": ["1"]
      }
    }],
    "people": [{
      "id": "9",
      "name": "DHH"
    }]
  }
}
```

It's easy to imagine a scenario in which resources that are members of the primary array are secondary resources for other members of the primary array (e.g. people + friends). In those scenarios, primary resources should not be duplicated as secondary resources in `linked`. In other words, a single canonical representation of a resource should continue to be returned with each response.

[Closes #74]