[Bridge][Propel1][Form] Model choice accept custom unique column

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

This PR permit to do :

```` php
<?php
public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder->add('my_field', 'model', array(
                        'class' => 'MyClassWithSlug', 
                        'index_property' => 'slug'
    ));
}
````

With this option you can hide internal ids to expose an unique column eg slug and render 

```` html
<select name="my_field">
      <option value="{THE_SLUG}">{THE_VALUE}</option>
</select>
````