[Suggestion] ActiveCollection

`1`. This will allow to do:

```php
$valid = $models->validate();
```

instead of

```php
$valid = true;
foreach ($models as $model) {
    $valid = $model->validate() && $valid;
}
```

`2`. This will allow to do:

```php
$success = $models->save();
```

instead of

```php
$success = true;
foreach ($models as $model) {
    $success = $model->save() && $success ;
}
```

`3`. This allow to do:

```php
$models->populate($_POST);
```

Also there can be other useful methods that work with collection. This addition allow to be DRY and KISS.