Sync links with the latest spec

Right now links are represented using the old syntax:

``` json
{
    "self": "http...",
    "foo": "bar"
}
```

The latest spec says it should be like this:

``` json
{
    "href": "http...",
    "meta": {
        "foo": "bar"
    }
}
```

Links can anyway be added as strings, in both spec versions and this is supported.
