Validation Attribute Labels

Making this blog post happen: http://forums.laravel.io/viewtopic.php?pid=14543#p14543

Added a fourth param that allows a keyval array with attribute labels in. Personal reason for the addition is to aid in multilingual validation (managed via a cms rather than just config files).

```php
$input = array(
		'user_email' => $user_email,
		'user_password' => $user_password
);
$rules = array(
		'user_email' => 'required|email|exists:users',
		'user_password' => 'required'
);
$friendly_names = array(
		'user_email' => 'Email Address',
		'user_password' => 'Password'
);

$validation = Validator::make( $input, $rules, null, $friendly_names );
```