Support eager loading for loop.

For example:
```php
$users = Sentry::getUserProvider()
    ->createModel()
    ->with('groups')
    ->limit(5)
    ->get();

foreach ($users as $user) {
    var_dump($user->email);
    var_dump($user->isSuperUser());
}
```

It's result with 1+1+**5** queries like this:
![screenshot from 2014-07-08 09 40 49](https://cloud.githubusercontent.com/assets/1529454/3504562/b5695c0e-0649-11e4-8df6-7e4ae1846b0a.png)