Allows soft models w/slug observer to be deleted+recreated w/same source property

Issue can be re-created with a fresh install of 1.8 develop:

Create a Soft Model:
```php
class Model_Test extends Orm\Model_Soft
{
	protected static $_properties = array(
		'id',
		'title',
		'slug',
		'deleted_at',
	);
	
	
	protected static $_observers = array(
		'Orm\\Observer_Slug' => array(
			'events' => array('before_insert', 'before_update'),
			'source' => 'title',
			'property' => 'slug',
		),
	);
}
```

Run a controller method with the following:
```php
		DBUtil::truncate_table('tests');

		$model = Model_Test::forge(array(
			'title' => 'a',
			)
		);
				
		$model->save();
		
		$model->delete();
		
		$model = Model_Test::forge(array(
			'title' => 'a',
			)
		);
		
		$model->save();
```

Exception will be thrown on duplicated row:
    Fuel\Core\Database_Exception [ 1062 ]:
    Duplicate entry 'a' for key 'slug' [ INSERT INTO `tests` (`title`, `slug`, `deleted_at`) VALUES ('a', 'a', null) ]

I have mitigated the issue by adding a boolean property to the slug observer class which disables the filter temporarily for soft models, while it builds the query to search for duplicate properties (title by default).