title: Select only one index of multiindex DataFrame
body: <p>I am trying to create a new DataFrame using only one index from a multi-indexed DataFrame.  </p>
  
  <pre><code>                   A         B         C
  first second                              
  bar   one     0.895717  0.410835 -1.413681
        two     0.805244  0.813850  1.607920
  baz   one    -1.206412  0.132003  1.024180
        two     2.565646 -0.827317  0.569605
  foo   one     1.431256 -0.076467  0.875906
        two     1.340309 -1.187678 -2.211372
  qux   one    -1.170299  1.130127  0.974466
        two    -0.226169 -1.436737 -2.006747
  </code></pre>
  
  <p>Ideally, I would like something like this:</p>
  
  <pre><code>In: df.ix[level="first"]
  </code></pre>
  
  <p>and:</p>
  
  <pre><code>Out:
  
                 A         B         C
  first                               
  bar        0.895717  0.410835 -1.413681
             0.805244  0.813850  1.607920
  baz       -1.206412  0.132003  1.024180
             2.565646 -0.827317  0.569605
  foo        1.431256 -0.076467  0.875906
             1.340309 -1.187678 -2.211372
  qux       -1.170299  1.130127  0.974466
            -0.226169 -1.436737 -2.006747
  `
  </code></pre>
  
  <p>Essentially I want to drop all the other indexes of the multi-index other than level <code>first</code>.  Is there an easy way to do this?</p>