Added string export of Query object & criteria

> Required by the (QueryBuilder API tests)[https://github.com/ezsystems/QueryBuilderBundle/tree/unit_tests/eZ/Publish/API/QueryBuilder/Tests]

Offers a human readable view of a query:

```php
$queryBuilder
    ->contentTypeIdentifier()->in( 'article', 'blog_post' )
    ->sectionId()->eq( 1 )
    ->textLineField( 'title' )->contains( 'service' )
    ->sortBy()
        ->contentName()->descending();
```

translates into:

```
contentTypeIdentifier IN (article, blog_post) AND sectionId = 1 AND title CONTAINS service
```

Implements __toString in Query and Criterion. SortClause not handled yet.