Adding type to links when necessary

Adds a type to the `links` key when the relationship name is different than the related object:

```
// Given:
Post = DS.Model.extend({
  comments: DS.hasMany('comment')
});

{
  "id": "1",
  "links": {
    "comments": [ "5", "12", "17", "20" ]
  }
}
```

Now when the name of the realtionship is different to the related type

```
// Given:
Post = DS.Model.extend({
  observations: DS.hasMany('comment')
});

{
  "id": "1",
  "links": {
    "observations": {
      "ids": [ "5", "12", "17", "20" ],
      "type": "comment"
    }
  }
}
```