Return correct results when using a BelongsToMany relationship

I was having a problem using the `find` method when using a `BelongsToMany` relationship. I would do the following in a controller:

```php
public function __construct(GamesRepository $repository)
{
		$this->repository  = $repository;
                // I only want the games of the current users to be available
		$this->repository->setItems(Auth::user()->student->games());
}
```

Then further down I'd do 

```php
$game = $this->repository->find(1);
var_dump($game->id); // Would return 4
```

Turns out the following query is executed:

```sql
select * from `games` inner join `game_player` on `games`.`id` = `game_player`.`game_id` where `game_player`.`player_id` = 1 and `games`.`id` = '1'
```

Which returns the following result:

```sql
id	company_id	status	actions_in_play	actions_market	created_at	updated_at	id	game_id	player_id	score	budget	actions
4	2	invitation	NULL	NULL	2014-12-11 08:13:34	2014-12-11 08:13:34	7	4	4	0	40	NULL
```

What this PR does is check if the `$items` is an instance of `BelongsToMany` and if so it will select only the columns from the relation  table, eg: `games.*`