Add a AddParameters method in the QueryBuilder

Hi,

On doctrine 2.2, when we were calling the method `setParameters` to set a bunch of parameters on the QueryBuilder, each parameters were added or modified if they were already existing. 

That is not the case since doctrine 2.3 : each calls to `setParameters` will wipe out the other already setted parameters. I think it is logic, but still is a huge BC break, which is not really mentionned in the UPGRADE file (only the once regarding the use of an ArrayCollection is mentionned). 

With this PR, I added a `addParameters` which allows to add several parameters in several calls. Here is a use case :

```php
<?php
// ... A repository
class UserRepository extends EntityRepository
{
     public function getUserWithSomething($criteriaA, $criteriaB, DateTime $since = null)
     {
         $qb = $this->createQueryBuilder('u');
         $qb->where($qb->expr()->andX($qb->expr()->eq('u.criteriaA', ':criteriaA'),
                                                       $qb->expr()->eq('u.criteriaB', ':criteriaB')));

         // wanna search for objects since a specific date
         if (null !== $since) {
             $qb = $this->addSince($qb, $since);
         }

         $qb->addParameters(new ArrayCollection(['criteriaA' => $criteriaA, 
                                                                       'criteriaB' => $criteriaB]));

         return $qb->getQuery()->execute();
     }

     public function addSince(QueryBuilder $qb, DateTime $since)
     {
         $qb = $qb->andWhere($qb->expr()->gte('u.date', ':since'));

         return $qb->setParameter('since', $since);
     }
}
```

So, as I was saying, until 2.2, you could use `setParameters` in the main method (`getUserWithSomething`) which was calling (or not) the submethod `addSince`, which could set parameters before calling the `setParameters` method. Now, you can't do that anymore : either you need to make several calls to `setParameter` :

```php
<?php
// ....
     $qb = $qb->setParameter('criteriaA', $criteriaA)
                   ->setParameter('criteraB', $criteriaB);
//...
```

Either, I guess, you need to first retrieve all the parameters and then add them by hand : 

```php
<?php
// ....
     $parameters = $qb->getParameters();
     $parameters->add(new Parameter('criteriaA', $criteriaA);
     $parameters->add(new Parameter('criteriaB', $criteriaB);

     $qb = $qb->setParameters($parameters);
//...
```

So that's why I propose this new method in the QueryBuilder. if I'm wrong anywhere, or need more information please feel free to comment. :)

Cheers.