title: QueryBuilder alias for expr() in select() method
body: <p>is there any option to set alias for  $qb->expr()->avg('e.value') in select() method of Doctrine2 query builder? </p>
  
  <p>I have e.g. this query:</p>
  
  <pre><code>$qb-&gt;select($qb-&gt;expr()-&gt;avg('e.value'), 'e someAlias')-&gt;from('Entity', 'e');
  </code></pre>
  
  <p>But the average value is indexed by integer in the result, like this:</p>
  
  <pre><code>array(
    0 =&gt; array(
     'someAlias' =&gt; Entity {},
     1 =&gt; 2.5255,
    ),
  );
  </code></pre>
  
  <p>Is it possible to change key of the average value to the defined string value?</p>