Skip keys with the @Ignore annotation in the FormExtractor

This small change allows you to skip keys from FormTypes by adding the @Ignore annotation.

Either put the annotation before the array key or array value:

```php
$builder->add('foo', 'bar', array('label' => /** @Ignore */ 'Foo'));
```
or 
```php
$builder->add('foo', 'bar', array(/** @Ignore */ 'label' => 'Foo'));
```

Works for FormTypes as well as Sonata Admin extensions.

It would be possible to ignore entire arrays (useful for ``choices`` for example) but this would mean duplicating code and therefore wasn’t included in the PR.

See #20