title: How do you shift Pandas DataFrame with a multiindex?
body: <p>With the following DataFrame, how can I shift the "beyer" column based on the index without having Pandas assign the shifted value to a different index value?</p>
  
  <pre><code>                  line_date  line_race  beyer
  horse                                        
  Last Gunfighter  2013-09-28         10     99
  Last Gunfighter  2013-08-18         10    102
  Last Gunfighter  2013-07-06          8    103
  .....
  Paynter          2013-09-28         10    103
  Paynter          2013-08-31         10     88
  Paynter          2013-07-27          8    100
  </code></pre>
  
  <p><code>df['beyer'].shift(1)</code> produces...</p>
  
  <pre><code>                  line_date  line_race  beyer  beyer_shifted
  horse                                                       
  Last Gunfighter  2013-09-28         10     99            NaN
  Last Gunfighter  2013-08-18         10    102             99
  Last Gunfighter  2013-07-06          8    103            102
  .....
  Paynter          2013-09-28         10    103             71
  Paynter          2013-08-31         10     88            103
  Paynter          2013-07-27          8    100             88
  </code></pre>
  
  <p>The problem is that Paynter was given a beyer that Last Gunfighter (his first record) was assigned. Instead I want it to go like this...</p>
  
  <pre><code>                  line_date  line_race  beyer  beyer_shifted
  horse                                                       
  Last Gunfighter  2013-09-28         10     99            NaN
  Last Gunfighter  2013-08-18         10    102             99
  Last Gunfighter  2013-07-06          8    103            102
  .....
  Paynter          2013-09-28         10    103            NaN
  Paynter          2013-08-31         10     88            103
  Paynter          2013-07-27          8    100             88
  </code></pre>