Add belongs_to relation type

Hi, i try realize BELONGS_TO relation type (analogue CActiveRecord::BELONGS_TO)
I have my doubts about the correctness. Can you help with the correction, if that is not

Example usage:

```PHP
    public function relations()
    {
        return [
            'mainPhoto'=> ['belongs_to', 'Photo', 'main_photo_id'],
        ];
    }
```

```JavaScript
> db.user.find().pretty()
{
        "_id" : ObjectId("528379b701c25e680800002c"),
        "role" : "registered",
        "main_photo_id" : ObjectId("5284e3a101c25ed008000032"),
}
```

```JavaScript
> db.photo.find().pretty()
{
        "_id" : ObjectId("5284e3a101c25ed008000032"),
        "filename" : "76e0d7ef165812335fc022dd035391ca.jpg",
}
```