[Suggestion] ActiveCollection

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