title: numpy diff on a pandas Series
body: <p>I want to use numpy.diff on a pandas Series. Am I right that this is a bug? Or am I doing it wrong?</p>
  
  <pre><code>In [163]: s = Series(np.arange(10))
  
  In [164]: np.diff(s)
  Out[164]: 
  0   NaN
  1     0
  2     0
  3     0
  4     0
  5     0
  6     0
  7     0
  8     0
  9   NaN
  
  In [165]: np.diff(np.arange(10))
  Out[165]: array([1, 1, 1, 1, 1, 1, 1, 1, 1])
  </code></pre>
  
  <p>I am using pandas 0.9.1rc1, numpy 1.6.1.</p>