Register admin elements via annotations

As you know (or maybe not) AdminBundle is based on  ``Admin\Manager``. It is an object that holds every single admin element. If admin element is not present in manager AdminBundle is not able to handle it properly. 
Up to now  there was only one way to add new elements to  ``Admin\Manager``. Element was added to Manager only if it was registered as a tagged service. 
From now it would be possible to add elements into manager via simple annotation. 

Example: 

```php
<?php

namespace FSi\FixturesBundle\Admin;

use FSi\Bundle\AdminBundle\Admin\ElementInterface;
use FSi\Bundle\AdminBundle\Annotation as Admin;

/**
 * @Admin\Element(group="structure")
 */
class MyAdminElement implements ElementInterface
{
}
```

This PR also introduce ``ElementFactory``, simple way to create admin element without worrying if element is "something aware" (for example FormAware or DataGridAware).

I think that there are a few things that could be improved here so dont hasitate to share your thoughts :)