title: How to slice a multi-indexed series
body: <p>I am creating a multi indexed series like this :</p>
  <pre><code>  index = [ ('America','California', 2000), ('America','California', 2010),
            ('Asia','Delhi', 2000), ('Asia','Delhi', 2010),
            ('Asia','HK', 2000), ('Asia','HK', 2010),
            ('America','Texas', 2000), ('America','Texas', 2010)
          ]
  
  populations = [33871648, 37253956,18976457, 19378102,30861820, 35145561,20851820, 25145561]
  
  new_index =  pd.MultiIndex.from_tuples(index)
  pop = pd.Series(populations, index=new_index).sort_index()
  </code></pre>
  <p>I am trying to retrieve values like this :</p>
  <pre><code>pop['America'] # works
  pop['America','Texas',2010] # works
  pop['America','Texas'] # works
  </code></pre>
  <h1>How to get for america, any city, but 2010?</h1>
  <pre><code>pop['America',:,2010] # not working
  </code></pre>
  <p>How to use slicing in situations like this ?</p>