Updated:
- Add aggregate support.
 eg: /article/aggregate?__count=id&__group_by=writer_id

 you will get this:

 {
  "__count": 2,
  "__model": "Article",
  "__ref": "/article/aggregate",
  "__type": "Aggregation",
  "Article": [
    {
      "__count_id": 2,
      "writer_id": 1
    },
    {
      "__count_id": 2,
      "writer_id": 2
    }
  ],
  "__total": 2
}