title: Apply difference function on each adjacent row value pandas
body: <p>I have a df like below and want to see if timestamps are always increasing.
  Basically a diff in each row for timestamp column. Add diff in a third column.</p>
  
  <pre><code>A,B,Timestamp
  5,58330831,1491375186654664218
  5,58330832,1491375186654673017
  5,58330833,1491375186654687270
  5,58330834,1491375186654696695
  5,58330835,1491375186654712416
  </code></pre>