title: How to get row delta value by pandas dataframe
body: <p>I got a dataframe combined by with <code>index = datetime</code> and <code>column = {'currentprice'}</code></p>
  
  <pre><code>index               currentPrice
  2015-03-26 10:09:01 75.75
  2015-03-26 10:11:57 75.70
  </code></pre>
  
  <p>Now I want to get the delta value during every 3 minutes(as an example), the I can get a dataframe like this:</p>
  
  <pre><code>index               delta
  2015-03-26 10:09:01 -0.05
  2015-03-26 10:11:57 0.10
  ...
  </code></pre>
  
  <p>What should I do?</p>