title: ReferenceMany doesn't work Doctrine ODM
body: <p>Hi everyBody i have actually a problem with doctrine (Symfony2 project)
  so in fact i have a document "Question" which references "$ref" tag which is a tag in the "Category" document but im getting error that i can't understand. i searched in the doctrine documentation but didn't find a solution, here is the code : </p>
  
  <pre><code>use Doctrine\ODM\MongoDB\Mapping\Annotations as MongoDB;
  
  
  /**
    *@MongoDB\Document(repositoryClass="ATS\QuizzBundle\Document\CategoryRepository")
  */
  
  class Category {
  
  /**
   * @MongoDB\Id
  */
  protected $id;
  
  /**
   * @MongoDB\String
  */
  protected $label;
  
  /**
   *@MongoDB\int
   *@ReferenceMany(targetDocument="Question")
  */
  protected $ref ;
  }
  </code></pre>
  
  <p>And i'm getting this error:</p>
  
  <pre><code>  [Semantical Error] The annotation "@MongoDB\Document" in class ATS\QuizzBun  
    dle\Document\Category was never imported. Did you maybe forget to add a "us  
    e" statement for this annotation?
  </code></pre>