title: Select rows of DataFrame with datetime index based on date
body: <p>From the following DataFrame with datetime index</p>
  
  <pre><code>                                       'A'
  2015-02-17 14:31:00+00:00           127.2801
  2015-02-17 14:32:00+00:00           127.7250
  2015-02-17 14:33:00+00:00           127.8010
  2015-02-17 14:34:00+00:00           127.5450
  2015-02-17 14:35:00+00:00           127.6300
  ...
  2016-02-17 20:56:00+00:00            98.0900
  2016-02-17 20:57:00+00:00            98.0901
  2016-02-17 20:58:00+00:00            98.1000
  2016-02-17 20:59:00+00:00            98.0500
  2016-02-17 21:00:00+00:00            98.1100
  </code></pre>
  
  <p>I want to select all rows with a certain date, e.g. 2015-02-17. </p>
  
  <p>Whats the best way to achieve that?</p>