title: Doctrine YAML generates Datetime instead of Timestamp
body: <pre><code>Category:    
    columns:
      id:
      type: integer(11)
      primary: true
      autoincrement: true
      unsigned: true
    name:
      type: string(255)
      notnull: true
  actAs:
    Timestampable:
      created:
        name: created_at
        type: timestamp
        format: Y-m-d H:i:s
      updated:
        name: updated_at
        type: timestamp
        format: Y-m-d H:i:s
  </code></pre>
  
  <p>Hi everyone!
  Here is my schema.yml and the problem is that when I run doctrine buil-all-reload it generates database but instead of timestamp it adds datetime. Why?<br>
  This migh be another question but since it is closely related I will ask here. I wanted to use timestamp because of <code>on update CURRENT_TIMESTAMP</code> Is there another way of doing such thing in Zend or Doctrine?</p>