Custom object actions stub generator

@sescandell @CayetanoSoriano @cedriclombardot This is ready to merge and tested, but before I merge this I'd like you to have a look at the code (in case I missed something).

In applications you often have many custom actions. For example, you can have an online shop with a customer ordering something. Your order will probably have multiple custom actions like:

* confirm -> confirms you received the order
* send -> set "is sent" to true
* cancel -> cancels order

To quickly manage many actions I've introduced a new builder called `custom` which main goal is to generate STUBs for custom actions.

Example config:
```yaml
generator: admingenerator.generator.doctrine
params:
    model:              Acme\ShopBundle\Entity\Order
    namespace_prefix:   Acme
    bundle_name:        ShopBundle
    fields: ~
    object_actions:
            confirm:
                label:    Confirm
                icon:     icon-ok
                route:    Acme_ShopBundle_Order_custom
                csrfProtected: true
                params:
                    title:  "Are you sure you want to confirm this order?"
                    pk:       "{{ Order.id }}"
                    action:   confirm
            cancel:
                label:    Cancel
                icon:     icon-remove
                route:    Acme_ShopBundle_Order_custom
                csrfProtected: true
                params:
                    title:  "Are you sure you want to cancel this order?"
                    pk:       "{{ Order.id }}"
                    action:   cancel
  
builders:
    list:
      params:
        title:    title.list
        display:  [ id, username, email, lastLogin, enabled, roles, groups ]
        actions:
            new: ~
        # object_actions: ~
        # removed object actions from here, so that list builder uses general params
        batch_actions:
            delete: ~

    custom:
      params:
        # object_actions: ~
        # removed object actions from here, so that list builder uses general params
```

Then you need to edit the custom controller in `Acme/ShopBundle/Controller/Order/CustomController.php` and customize `executeConfirm` and `executeCancel` functions like that:

```php
<?php

namespace Acme\ShopBundle\Controller\Order;

use Admingenerated\AcmeShopBundle\BaseOrderController\CustomController as BaseCustomController;

class CustomController extends BaseCustomController
{
    public function executeConfirm($Order)
    {
        $em = $this->get('doctrine')->getManager();
        $Order->setConfirmed(true);
        $em->persist($Order);
        $em->flush();
    }
    
    public function executeCancel($Order)
    {
        $em = $this->get('doctrine')->getManager();
        $Order->setCanceled(true);
        $em->persist($Order);
        $em->flush();
    }
}
```

And... thats all! It works!