title: UNION query in propel ORM v1.6
body: <p>I am converting some existing queries into propel ORM.
  Now i am stuck with converting following similar query in propel.</p>
  
  <pre><code>(SELECT name,address,phone FROM table_1)
  UNION
  (SELECT name,address,phone FROM table_2)
  UNION
  (SELECT name,address,phone FROM table_3)
  </code></pre>
  
  <p>What would be the proper propel implementation for the above query?</p>