title: How to aggregate pandas Dataframe by day
body: <p>I have got the following pandas DataFrame: </p>
  
  <pre><code>time,                value
  2018-08-02 09:19:37, 2
  2018-08-02 09:19:47, 3
  2018-08-02 09:19:57, 6
  ......
  2018-08-03 04:49:27, 2
  2018-08-03 04:49:37, 4
  2018-08-03 04:49:47, 5
  </code></pre>
  
  <p>I would like to build an output dataFrame as in the following: </p>
  
  <pre><code>time,                value
  2018-08-02 11:59:59, AVG(2+3+6+..)
  2018-08-03 11:59:59, AVG(2+4+5+..)
  </code></pre>
  
  <p>Please any help would very appreciated. Many Thanks.</p>