title: Add rate of change column to Pandas DataFrame
body: <p>I have the following Pandas DataFrame:</p>
  <pre><code>    lastrun                                value 
  0   2013-10-24 13:10:05+00:00              55376
  1   2013-10-24 14:10:32+00:00              56738
  2   2013-10-24 15:52:31+00:00              58239
  3   2013-10-24 23:52:09+00:00              59981
  4   2013-10-25 00:52:04+00:00              61001
  </code></pre>
  <p>I would like to add a column into the dataframe with the rate of change, to get:</p>
  <pre><code>    lastrun                                value    change 
  0   2013-10-24 13:10:05+00:00              55376    NaN
  1   2013-10-24 14:10:32+00:00              56738    1362
  2   2013-10-24 15:52:31+00:00              58239    1501
  3   2013-10-24 23:52:09+00:00              59981    1742
  4   2013-10-25 00:52:04+00:00              61001    1020
  </code></pre>
  <p>I know the percentage change can be found using <code>pct_change()</code> but I don't know how to include it as a new column in the existing DataFrame and I'm looking for the real value in change and not the % change.</p>
  <p>What is the most effective way to achieve this in pandas (or python, numpy)?</p>