title: Find integer index of rows with NaN in pandas dataframe
body: <p>I have a pandas DataFrame like this:</p>
  
  <pre><code>                    a         b
  2011-01-01 00:00:00 1.883381  -0.416629
  2011-01-01 01:00:00 0.149948  -1.782170
  2011-01-01 02:00:00 -0.407604 0.314168
  2011-01-01 03:00:00 1.452354  NaN
  2011-01-01 04:00:00 -1.224869 -0.947457
  2011-01-01 05:00:00 0.498326  0.070416
  2011-01-01 06:00:00 0.401665  NaN
  2011-01-01 07:00:00 -0.019766 0.533641
  2011-01-01 08:00:00 -1.101303 -1.408561
  2011-01-01 09:00:00 1.671795  -0.764629
  </code></pre>
  
  <p>Is there an efficient way to find the "integer" index of rows with NaNs? In this case the desired output should be <code>[3, 6]</code>.</p>