title: Timestampable Trait of Gedmo Doctrine extension throws error
body: <p>I got the following entity</p>
  
  <pre><code>use Gedmo\Timestampable\Traits\TimestampableEntity;
  
  /**
   * @ORM\Entity
   * @ORM\Table(name="users")
   * @UniqueEntity(fields="email", message="Email already taken")
   */
  class User
  {
      /**
       * Hook timestampable behavior
       * updates createdAt, updatedAt fields
       */
      use TimestampableEntity;
  </code></pre>
  
  <p>...</p>
  
  <p>if i run</p>
  
  <pre><code>$ php app/console doctrine:mapping:info
  </code></pre>
  
  <p>i get this error</p>
  
  <pre><code>found 8 mapped entities:
  [OK]   Gedmo\Translatable\Entity\MappedSuperclass\AbstractPersonalTranslation
  [OK]   Gedmo\Translatable\Entity\MappedSuperclass\AbstractTranslation
  [OK]   Gedmo\Translatable\Entity\Translation
  [OK]   Gedmo\Loggable\Entity\MappedSuperclass\AbstractLogEntry
  [OK]   Gedmo\Loggable\Entity\LogEntry
  [OK]   Gedmo\Tree\Entity\MappedSuperclass\AbstractClosure
  [OK]   Psren\CmsBundle\Entity\Group
  
  
  
    [Doctrine\Common\Annotations\AnnotationException]                                                                                                                                                    
    [Semantical Error] The annotation "@Gedmo\Timestampable" in property Psren\CmsBundle\Entity\User::$createdAt was never imported. Did you maybe forget to add a "use" statement for this annotation?
  </code></pre>
  
  <p>the annotation @Gedmo\Timestampable was defined in the trait of the extension. so it must have been loaded.</p>
  
  <p>what went wrong?
  thanks for help.</p>