title: Propel ORM UNION query
body: <p>I have three criteria objects $c1, $c2, $c3</p>
  
  <p>I want to return the union set of all three i.e.: ($c1 || $c2 || $c3) 
  I have a function that expects a Criteria object, so I need to be able to return a criteria that represents the UNION of $c1, $c2 and $c3.</p>
  
  <p>Does anyone know how to achieve that?</p>