[GH-590] DQL Query: process ArrayCollection values to ease development

This issue is created automatically through a Github pull request on behalf of michaelperrin:

Url: https://github.com/doctrine/doctrine2/pull/590

Message:

I added some code to ease "where in" parameter binding.

As you know, when a where condition is added, the object itself can be passed as a parameter and it's id is automatically retrieved:

```php
$queryBuilder = $this
->where('model.category = :category')
->setParameter('category', $category)
;
```
Where `$category` is an object.

But it doesn't work for collections:
```php
$queryBuilder = $this
->where('model.category IN (:categories)')
->setParameter('categories', $categories)
;
```

Where categories is an `ArrayCollection` object (retrieved from a many to one relation for instance).

This doesn't work in the current version of Doctrine, and my PR solved that.

So far, the only solution is to do the following:

```php
$categoryIds = array();

foreach ($categories as $category)

$queryBuilder = $this
->where('model.category IN (:category_ids)')
->setParameter('category_ids', $categoryIds)
;
```

And this is pretty borring when you have to do it several times for several entities.

Note that I didn't add any unit test for this feature. Can you explain me where I should add the test?

Thanks!