title: how to take average of the values per three days in a timeseries data
body: <p>I have a time-series data that look like this:</p>
  
  <pre><code>datetime    | value
  2016-01-01  | 1
  2016-01-02  | 14
  2016-01-03  | 3
  2016-01-04  | 15
  2016-01-05  | 5
  2016-01-06  | 4
  2016-01-07  | 7
  2016-01-08  | 15
  </code></pre>
  
  <p>What I want to obtain is the average per three days (as well as per a week) and keep the last day as the index like this:</p>
  
  <pre><code>datetime    | value
  2016-01-03  | 6
  2016-01-06  | 8
  2016-01-08  | 11
  </code></pre>
  
  <p>You may notice that the remaining 2 records need to be just averaged. Is there an handy way of doing this either in pandas <code>dataframe</code> or graphlab <code>SFrame</code>? If someone can share a relevant resource, that would be great!</p>
  
  <p>I appreciate any help!</p>