title: Counting frequency of values by date using pandas
body: <p>Let's suppose I have following Time Series:</p>
  
  <pre><code>Timestamp              Category
  2014-10-16 15:05:17    Facebook
  2014-10-16 14:56:37    Vimeo
  2014-10-16 14:25:16    Facebook
  2014-10-16 14:15:32    Facebook
  2014-10-16 13:41:01    Facebook
  2014-10-16 12:50:30    Orkut
  2014-10-16 12:28:54    Facebook
  2014-10-16 12:26:56    Facebook
  2014-10-16 12:25:12    Facebook
  ...
  2014-10-08 15:52:49    Youtube
  2014-10-08 15:04:50    Youtube
  2014-10-08 15:03:48    Vimeo
  2014-10-08 15:02:27    Youtube
  2014-10-08 15:01:56    DailyMotion
  2014-10-08 13:27:28    Facebook
  2014-10-08 13:01:08    Vimeo
  2014-10-08 12:52:06    Facebook
  2014-10-08 12:43:27    Facebook
  Name: summary, Length: 600
  </code></pre>
  
  <p>I would like to make a count of each category (Unique Value/Factor in the Time Series) per week and year.</p>
  
  <pre><code>Example:
  
      Week/Year      Category      Count
      1/2014         Facebook      12
      1/2014         Google        5
      1/2014         Youtube       2
  ...    
      2/2014         Facebook      2
      2/2014         Google        5
      2/2014         Youtube       20
  ...
  </code></pre>
  
  <p>How can this be achieved using Python pandas?</p>