title: group by pandas dataframe and select latest in each group
body: <p>How to group values of pandas dataframe and select the latest(by date) from each group? </p>
  
  <p>For example, given a dataframe sorted by date:</p>
  
  <pre><code>    id     product   date
  0   220    6647     2014-09-01 
  1   220    6647     2014-09-03 
  2   220    6647     2014-10-16
  3   826    3380     2014-11-11
  4   826    3380     2014-12-09
  5   826    3380     2015-05-19
  6   901    4555     2014-09-01
  7   901    4555     2014-10-05
  8   901    4555     2014-11-01
  </code></pre>
  
  <p>grouping by id or product, and selecting the earliest gives:</p>
  
  <pre><code>    id     product   date
  2   220    6647     2014-10-16
  5   826    3380     2015-05-19
  8   901    4555     2014-11-01
  </code></pre>