title: Randomly insert NA's values in a pandas dataframe
body: <p>How can I randomly insert <code>np.nan</code>'s in a DataFrame ?
  Let's say I want 10% null values inside my DataFrame. </p>
  
  <p>My data looks like this : </p>
  
  <pre><code>df = pd.DataFrame(np.random.randn(5, 3), 
                    index=['a', 'b', 'c', 'd', 'e'],
                    columns=['one', 'two', 'three'])
  
          one       two     three
  a  0.695132  1.044791 -1.059536
  b -1.075105  0.825776  1.899795
  c -0.678980  0.051959 -0.691405
  d -0.182928  1.455268 -1.032353
  e  0.205094  0.714192 -0.938242
  </code></pre>
  
  <p>Is there an easy way to insert the null values?</p>