Enhance Auth Form adapter

As of now it is very hard to bind a model to a login form when using Auth/Form as the Form adapter has no way to read sub array data. This enhancement makes the Form adapter check if a sub array matching the models name exists and if so use it for `Auth::check()`
```php
<?= $this->form->create($user); ?>
<?= $this->form->text('user.username'); ?>
<?= $this->form->password('user.password'); ?>
<?= $this->form->end(); ?>
```