Support one-to-one relationships

[toddkazakov]

Currently all of the relationships are one-to-many. This can be seen in the following example:

    "relationships": {
        "source": {
          "links": {
            "self": "/annotate/361878dab8298d35e36c5e032c2255c998080743"
          },
          "data": [
            {
              "type": "source",
              "id": "361878dab8298d35e36c5e032c2255c998080743"
            }
          ]
        }
    }

As you can see data is an array, but it should be an object for one-to-one relationships.