add summary logging

Example output:
```js
{
    "level": 40,
    "levelname": "INFO",
    "name": "fxa.summary",
    "op": "fxa.summary",
    "pid": 9780,
    "hostname": "Ghul",
    "time": "2014-05-09T18:25:51.896Z",
    "v": 1,
    "code": 200,
    "errno": 0,
    "path": "/v1/client/dcdb5ae7add825d2",
    "agent": "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:32.0) Gecko/20100101 Firefox/32.0",
    "t": 7,
    "client_id": "dcdb5ae7add825d2"
}
```

@fmarier r? @kparlante look right?

fixes #42