add api router 'game'

change log:

**first**, rename 'itemId' to 'item' in game model. In mongodb, it store a item id. In game router, it store a completely item infomation, such as title, price and so on, like this:
```
{
    "data": {
        "_id": "53fbf18806dfc7351d689291",
        "item": {
            "_id": "53fbf0c82d8b94081d78012d",
            "title": "shit",
            "price": 100,
            "__v": 0
        },
        "capacity": 100,
        "status": "inited",
        "countdown": 100,
        "__v": 0
    }
}
```
**then**, add api game router:
```
GET /api/games // return all games
POST /api/games // create a game, params see lib/models/game.js
GET /api/games/:gameId // return a specific game
DELETE /api/games/:gameId // delete a specific game
```
@fuqcool