title: pandas: for each row in df copy row N times with slight changes
body: <p>So I have a DataFrame like this:</p>
  
  <pre><code>   N    start
  1  1    08/01/2014 9:30:02
  2  1    08/01/2014 10:30:02 
  3  2    08/01/2014 12:30:02
  4  3    08/01/2014 4:30:02
  </code></pre>
  
  <p>and I need to duplicate each row N times, adding one hour to start each time, like this:</p>
  
  <pre><code>   N    start
  1  1    08/01/2014 9:30:02
  2  1    08/01/2014 10:30:02 
  3  2    08/01/2014 12:30:02
  3  2    08/01/2014 13:30:02
  4  3    08/01/2014 4:30:02
  4  3    08/01/2014 5:30:02
  4  3    08/01/2014 6:30:02
  </code></pre>
  
  <p>how can I do it within pandas?</p>