title: pandas: set values with (row, col) indices
body: <p><code>pandas</code> offers the ability to look up by lists of row and column indices,</p>
  
  <pre><code>In [49]: index = ['a', 'b', 'c', 'd']
  
  In [50]: columns = ['one', 'two', 'three', 'four']
  
  In [51]: M = pandas.DataFrame(np.random.randn(4,4), index=index, columns=columns)
  
  In [52]: M
  Out[52]: 
          one       two     three      four
  a -0.785841 -0.538572  0.376594  1.316647
  b  0.530288 -0.975547  1.063946 -1.049940
  c -0.794447 -0.886721  1.794326 -0.714834
  d -0.158371  0.069357 -1.003039 -0.807431
  
  In [53]: M.lookup(index, columns) # diagonal entries
  Out[53]: array([-0.78584142, -0.97554698,  1.79432641, -0.8074308 ])
  </code></pre>
  
  <p>I would like to use this same method of indexing to set <code>M</code>'s elements.  How can I do this?</p>