Added a none function that guarantees an empty set.

Hi,

I added a function.

```php
ORM::for_table('people')->none()->find_many();
```

That is guaranteed to return an empty set. This code devolves down to the following query:

```sql
SELECT * FROM `person` WHERE 0;
```

The reasons for having a dedicated function are:

- Syntatic sugar: `none` or a similarly named function makes more sense than `where_raw(0)`
- Possibility to optimise (by not actually executing the query)

An example use of this code in Paris would be:

```php
class User extends Model {

    public static function has_cookie($orm, $cookie_name) {
        if (isset($_COOKIE[$cookie_name]))  {
            $cookie = $_COOKIE[$cookie_name];
            return $orm->where('cookie', $cookie);
        }
        // Return empty set
        //return $orm->where_raw(0);
        return $orm->none();
    }

}

$users = Model::Factory('user')->filter('has_cookie', 'user')->find_many();
```