Add support for json output for statuses

There were a couple of pull requests #5 and #7 for this support, but none of them came with tests. This pull request does.

The json schema is the same as #5, but you can put `Accept: application/json` to get json result.

`GET "/" Accept: application/json`:
```json
{
  "status": "up",
  "services": [
    {
      "name": "Application",
      "description": "Main application",
      "status": "up",
      "last_seen": "2012-10-24T06:05:12Z"
    }
  ]
}
```