title: Group DatetimeIndex by day with pandas
body: <p>I have indexed my dataframe to a DateTimeIndex like so:</p>
  
  <pre><code>timstamp                 _id
  2018-05-09 16:56:40.940  somedata1
  2018-05-09 16:54:03.959  somedata2
  2018-05-10 16:53:42.975  somedata3
  2018-05-11 16:52:44.897  somedata4
  2018-05-11 16:46:35.902  somedata5
  </code></pre>
  
  <p>And I would like to show the frequency of each of the dates like so:</p>
  
  <pre><code>day                      count
  2018-05-09               2
  2018-05-10               1
  2018-05-11               2
  </code></pre>
  
  <p>Thanks in advance!</p>