title: Add column with number of days between dates in DataFrame pandas
body: <p>I want to subtract dates in 'A' from dates in 'B' and add a new column with the difference.</p>
  
  <pre><code>df
            A        B
  one 2014-01-01  2014-02-28 
  two 2014-02-03  2014-03-01
  </code></pre>
  
  <p>I've tried the following, but get an error when I try to include this in a for loop...</p>
  
  <pre><code>import datetime
  date1=df['A'][0]
  date2=df['B'][0]
  mdate1 = datetime.datetime.strptime(date1, "%Y-%m-%d").date()
  rdate1 = datetime.datetime.strptime(date2, "%Y-%m-%d").date()
  delta =  (mdate1 - rdate1).days
  print delta
  </code></pre>
  
  <p>What should I do?</p>