title: Doctrine orm:generate-proxies throwing "Can't instantiate custom generator"
body: <p>Doctrine 2.5. When trying to generate proxies manually with</p>
  
  <pre><code>doctrine orm:generate-proxies
  </code></pre>
  
  <p>an exception is thrown:</p>
  
  <blockquote>
    <p>[Doctrine\ORM\ORMException]<br>
    Can't instantiate custom generator : MyBundle\MyCustomGenerator</p>
  </blockquote>
  
  <p>I have a custom generator defined which works correctly:</p>
  
  <pre><code>/**
   * @ORM\Column(type="string")
   * @ORM\Id
   * @ORM\GeneratedValue(strategy="CUSTOM")
   * @ORM\CustomIdGenerator(class="MyBundle\MyCustomGenerator")
   */
  protected $id;
  </code></pre>
  
  <p>Also, instantiating the generator manually, like</p>
  
  <pre><code>$a = new MyBundle\MyCustomGenerator();
  </code></pre>
  
  <p>works. But for a reason the Doctrine Console throws the above exception.</p>
  
  <p>I've tried to debug and check what's going on.<br>
  The exception is defined in <a href="https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php#L704" rel="noreferrer"><code>completeIdGeneratorMapping</code></a> of <code>ClassMetadataFactory</code>. I checked that <code>$definition['class']</code> stores the name of my custom generator: <code>MyBundle\MyCustomGenerator</code>. But still, Doctrine can't find the class.<br>
  Thought I should add the definition to <code>cli-config.php</code> as <a href="http://doctrine-orm.readthedocs.org/en/latest/reference/tools.html#doctrine-console" rel="noreferrer">explained in the doc</a>, with <code>use MyBundle</code> or <code>use MyBundle\MyCustomGenerator</code>, but it did not work - still the same exception was thrown.</p>
  
  <p>Any ideas how should I make the Doctrine Console aware of my custom ID generator?</p>