[reporter] Add `pending` field for json reporter

The json reporter output now looks like,

```json
{
  "stats": {
    "suites": 1,
    "tests": 2,
    "passes": 1,
    "pending": 1,
    "failures": 0,
    "start": "2014-09-12T04:53:21.907Z",
    "end": "2014-09-12T04:53:21.910Z",
    "duration": 0
  },
  "tests": [
    {
      "title": "GET request",
      "fullTitle": "/songs GET request",
      "duration": 0,
      "err": {}
    },
    {
      "title": "GET response",
      "fullTitle": "/songs GET response",
      "err": {}
    }
  ],
  "pending": [
    {
      "title": "GET response",
      "fullTitle": "/songs GET response",
      "err": {}
    }
  ],
  "failures": [],
  "passes": [
    {
      "title": "GET request",
      "fullTitle": "/songs GET request",
      "duration": 0,
      "err": {}
    }
  ]
}
```