Leaderboard endpoints

Most of the work that I've done here is to retrieve the scores for individual leaderboards of a game, which is the `GET /api/v1/game/<game>/board/<board>` endpoint. For now, the `GET /api/v1/game/<game>/board` does not include the default output of the specific endpoint. (Only shows the name and the slug) https://github.com/cvan/galaxy-api/issues/62

For the `GET /api/v1/game/<game>/board/<board>` endpoint:

**Optional Parameters**
`sort`: `asc` or `desc` (default: `desc`)
`friendsOnly`: `true` or `false` (default: `false`)
 `_user`: user's ssa token. Required when `friendsOnly` is true. The code will checks the token when this parameter is specified, no matter `friendsOnly` is true or false
`page`: Page number. Defaults to 0
`limit`: Number of scores per page. Defaults to 10

See https://github.com/soedar/galaxy-api/blob/master/views/game/board.js#L194 for page start/stop calculation

**Sample Output**
```javascript
[
    {
        "user": {
            "avatar": "http://www.gravatar.com/avatar/52ee5da3a600790b01386b551ec2e36a",
            "username": "1234",
            "id": "1234"
        },
        "score": "200"
    },
    {
        "user": {
            "avatar": "http://www.gravatar.com/avatar/8c5a1bf958ec4bbdfafc8a2fb4614b75",
            "username": "soedar.sg",
            "id": "b27911cf-7fe7-4c95-80e8-0db204e571cf"
        },
        "score": "10"
    }
]
```