title: GQL: Find all entities that contain a substring
body: <p>I have some entities that have a StringProperty and I would like to query for all the entities that match a substring. Is there a way to do that using just GQL?</p>
  
  <p>For example, if my datastore looks like this:</p>
  
  <pre><code>ID/Name     question_text
  --------------------------------------------------------------
  3001        I like to eat chicken.
  3020        I only like to eat chicken that is deep fried.
  3045        I like filet mignon.
  3052        I like cheese.
  </code></pre>
  
  <p>What would the GQL query be to find all the entities that contain 'chicken' in question_text?</p>