Add thumbnailing functionality

Thought this would be really useful and also am using it. I think an easier approach like height/width is better than a a more complex approach like frozennode has.

Tell me what you think about it.

usage:
```php
    $repo->uploads('image')
        ->to('img/gallery')
        ->keepNames()
        ->allowOverwrite()
        ->addThumbnail(300, null, '_tn');
```