title: Summing column values based on multiindex
body: <p>I have a multiindex dataframe which for simplicity is as follows;</p>
  
  <pre><code>                      Values    
  01-01-2010 Belgium    1    
             Belgium    7
             UK         3 
             UK         4
             UK         2
             France     1
             France     3
  02-01-2010 Belgium    4
             UK         7
             UK         10
             UK         2
             France     4
  
  </code></pre>
  
  <p>I need to try and sum the value for each country for each day.
  The actual dataframe has approx 10 years of data and 40 countries. </p>
  
  <p>Is there any simple way of using the resample() function to do this?
  I can't seem to get one working with the multiindex. I could perhaps convert the countries back to a column?</p>
  
  <p>Any help much appreciated. </p>