title: Repurchase count with pandas
body: <p>I have such table:</p>
  
  <pre><code>Date       Buyer_id
  11.11.2016  1
  11.11.2016  2
  11.11.2016  2
  13.12.2016  1
  13.12.2016  3
  13.12.2016  4
  14.12.2016  3
  14.12.2016  1
  </code></pre>
  
  <p>I need to calculate in pandas Repurchase number
  So i need number of people who visited shop in <strong>different</strong> days (it's important).
  So buyers #1,3 are good, buyer #2,4 are bad.</p>
  
  <p>result should be:
  good_buyers = 2
  bad_buers = 2</p>