[HAPI JSON] Support severity

Accepts `severity` as `$.body.severity` like:

```json
{
  "type": "event",
  "body": {
    "id": 1,
    "timestamp": 1407824772.939664125,
    "hostName": "www.example.com",
    "content": "Error!",
    "severity": "error"
  }
}
```

Here is a list of available severity:

  * `unknown`
  * `info`
  * `warning`
  * `error`
  * `critical`
  * `emergency`

It may be better that we move `std::string` ->  `TriggerSeverityType` converter to some common location.