title: Get weekday/day-of-week for Datetime column of DataFrame
body: <p>I have a DataFrame <code>df</code> like the following (excerpt, 'Timestamp' are the index):</p>
  
  <pre><code>Timestamp              Value
  2012-06-01 00:00:00     100
  2012-06-01 00:15:00     150
  2012-06-01 00:30:00     120
  2012-06-01 01:00:00     220
  2012-06-01 01:15:00      80
  ...and so on.
  </code></pre>
  
  <p>I need a new column <code>df['weekday']</code> with the respective weekday/day-of-week of the timestamps.</p>
  
  <p>How can I get this?</p>