title: Shift time series with missing dates in Pandas
body: <p>I have a times series with some missing entries, that looks like this:</p>
  
  <pre><code>date     value
  ---------------
  2000       5
  2001      10
  2003      8
  2004      72
  2005      12
  2007      13
  </code></pre>
  
  <p>I would like to do create a column for the "previous_value". But I only want it to show values for consecutive years. So I want it to look like this:</p>
  
  <pre><code>date     value    previous_value
  -------------------------------
  2000       5        nan
  2001      10         5
  2003      8         nan
  2004      72         8
  2005      12        72
  2007      13        nan
  </code></pre>
  
  <p>However just applying pandas shift function directly to the column 'value' would give 'previous_value' = 10  for 'time' = 2003, and 'previous_value' = 12 for 'time' = 2007.</p>
  
  <p>What's the most elegant way to deal with this in pandas? (I'm not sure if it's as easy as setting the 'freq' attribute).</p>