title: getting seconds from numpy timedelta64
body: <p>I have a datetime index in pandas</p>
  
  <pre><code>index = np.array(['2013-11-11T12:36:00.078757888-0800',
                    '2013-11-11T12:36:03.692692992-0800',
                    '2013-11-11T12:36:07.085489920-0800',
                    '2013-11-11T12:36:08.957488128-0800'], dtype='datetime64[ns]')
  </code></pre>
  
  <p>I want to calculate the time difference in seconds. The way I came up with is:</p>
  
  <pre><code>diff(index).astype('float64')/1e9
  </code></pre>
  
  <p>is there a better/cleaner way?</p>