Add image form type

Displays a file input with an imagine thumbnail. Only ```image_path``` and ```image_filter``` options are mandatory.

```
 ->add('image', 'liip_imagine_image', array(
    'image_path' => '/uploads/my-image.jpg',
    'image_filter' => 'my_thumb',
    'image_attr' => array('class' => 'thumbnail'),
    'link_url' => '/uploads/my-image.jpg'
    'link_attr' => array('class' => 'link')
))
```

I often need this and I  think it would be nice to make it available in the bundle.