Fueling Stop Resource

I've changed the schema at some points to keep it consistent across resources (compare to #175):
* lowercase keys
* no user for creation
* value/unit objects
* embedded users/sensors in response
* ISO8601 datetimes
* numbers are numbers

##### Creation

```json
{
    "fuelType": "diesel",
    "car": "52f646df44aee65b2833134d",
    "cost": {
        "value": 123,
        "unit": "Euro"
    },
    "mileage": {
        "value": 123,
        "unit": "km"
    },
    "volume": {
        "value": 123,
        "unit": "l"
    },
    "time": "2014-02-08T14:55:14Z",
    "missedFuelStop": false
}
```

##### GET /user/:name/fuelings
```json
{
    "fuelings": [
        {
            "car": {
                "properties": {
                    "constructionYear": 2012,
                    "fuelType": "gasoline",
                    "id": "52f646df44aee65b2833134d",
                    "manufacturer": "BMW",
                    "model": "R 1200 GS ADV"
                },
                "type": "car"
            },
            "cost": {
                "unit": "Euro",
                "value": 123
            },
            "created": "2014-02-08T15:36:56Z",
            "fuelType": "diesel",
            "id": "52f64f1844aec2f61806ca66",
            "mileage": {
                "unit": "km",
                "value": 123
            },
            "missedFuelStop": false,
            "modified": "2014-02-08T15:36:56Z",
            "time": "2014-02-08T14:55:14Z",
            "user": {
                "created": "2014-02-08T14:55:14Z",
                "mail": "envirocar@autermann.org",
                "modified": "2014-02-08T14:55:14Z",
                "name": "auti"
            },
            "volume": {
                "unit": "l",
                "value": 123
            }
        }
    ]
}
```

##### GET /user/:name/fuelings/:id
```json
{
    "car": {
        "properties": {
            "constructionYear": 2012,
            "fuelType": "gasoline",
            "id": "52f646df44aee65b2833134d",
            "manufacturer": "BMW",
            "model": "R 1200 GS ADV"
        },
        "type": "car"
    },
    "cost": {
        "unit": "Euro",
        "value": 123
    },
    "created": "2014-02-08T15:36:56Z",
    "fuelType": "diesel",
    "id": "52f64f1844aec2f61806ca66",
    "mileage": {
        "unit": "km",
        "value": 123
    },
    "missedFuelStop": false,
    "modified": "2014-02-08T15:36:56Z",
    "time": "2014-02-08T14:55:14Z",
    "user": {
        "created": "2014-02-08T14:55:14Z",
        "mail": "envirocar@autermann.org",
        "modified": "2014-02-08T14:55:14Z",
        "name": "auti"
    },
    "volume": {
        "unit": "l",
        "value": 123
    }
}
```