Allow deleting new embedded form without save the parent form

When you embed a relation in a form, if you add a new object in the form, you cannot delete it until you save the form and delete the item. This is very annoying, because if you click by mistale on Add you need to create the object, save and delete afterwards.

There is a new option for embedRelation name 'add_delete_empty' to add a delete checkbox that allow to delete the newly created form. It defaults to false in order to not change the actual behavior.
There is new options also to change the behavior of the delete checkbox:
   - delete_alert_text    : corresponds to alert_text
   - delete_hide_parent   : corresponds to hide_parent
   - delete_parent_level  : corresponds to parent_level

The new checkbox appears in the same place as the delete checkbox for regular objects:

![new_checkbox](https://f.cloud.github.com/assets/664229/85998/2739352a-6484-11e2-93d6-d3f39a6ade15.jpg)