sonata_type_model_autocomplete form type (autocompletion for too many items)

``sonata_type_model_autocomplete`` is additional model form type for ``sonata_type_model`` and ``sonata_type_model_list`` for handling many-to-one and many-to-many relations.

It's created because ``sonata_type_model`` can't be used for too many items and ``sonata_type_model_list`` is not ideal for some use cases. 

This form type is using select2 ajax feature. 

Usage is very simple:

```php
    // e-mail is a property from related User entity, on which we will filter the data
    $formMapper->add('user', 'sonata_type_model_autocomplete', array('property'=>'eMail'));

    // or if you need to change the query, you can specify callback option
    $formMapper->add('user', 'sonata_type_model_autocomplete', 
        array(
            'property'=>'eMail', 
            'callback'=>function($queryBuilder, $alias, $property, $value){
                        $queryBuilder->andWhere($alias.'.enabled=1');
                    }));

    // there are additional options, like `url`, `placeholder`, `minimum_input_length`, `items_per_page` ... 
```

Note: I had to rename `collection` form type to `sonata_type_native_collection`, becuase `collection` is used inside this form type and I had to avoid of the add/delete buttons from native `collection` type.
I tried to avoid any BC break here, so you can use `collection` form type in FormBuilder without any change in your code.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1172, #1758, partially #633
| License       | MIT
|DoctrimeOrmAdmin PR | sonata-project/SonataDoctrineORMAdminBundle#296

TODO:
- [x] many-to-many support
- [x] fix collection output
The ``multiple`` options work only with DoctrineORM.
You must change code for call the method "find" to each iteration, it's slow but there is no method to find several identifiers.

What do you think @rande ?