title: Group result by 15 minutes time interval in MongoDb
body: <p>I have a "status" collection like this strcture -</p>
  
  <pre><code>{
      _id: ObjectId("545a0b63b03dbcd1238b4567"),
      status: 1004,
      comment: "Rem dolor ipsam placeat omnis non. Aspernatur nobis qui nisi similique.",
      created_at: ISODate("2014-11-05T11:34:59.804Z")
  },
  {
      _id: ObjectId("545a0b66b03dbcd1238b4568"),
      status: 1001,
      comment: "Sint et eos vero ipsa voluptatem harum. Hic unde voluptatibus et blanditiis quod modi.",
      created_at: ISODate("2014-11-05T11:35:02.814Z")
  }
  ....
  ....
  </code></pre>
  
  <p><strong>I need to get result grouped by 15 minutes interval from that collection.</strong></p>