Add the possibility to change the default label of a embedded realtion from new<RelationName> to an arbitrary name

If you are a non english speaker (or your customer), when you embed a relation in a form always get **new<RelationName>** as a label in the related form when you allow to add empty objects. In this case, you can pass a new option named **'empty_name'** with the label that you want for the new object. 


Example of use:
```php
    $this->embedRelation('ConcursoPatrocinador',
                         array('title' => 'Patrocinadores',
                               'empty_name' => 'Nuevo patrocinador',
                               'add_link'   => 'Añadir patrocinador,
                               'add_empty' => true));
```

without empty_name:
![default_label](https://f.cloud.github.com/assets/664229/78843/66ed86a4-6195-11e2-9fbc-b38d2bd84602.jpg)

with empty_name:
![nuveo_pat](https://f.cloud.github.com/assets/664229/78834/322e4322-6195-11e2-8c9a-5177353ce891.jpg)