title: How to select multiple column names in specific one level with Python pandas
body: <p>There is a DataFrame which has a MultipleIndex as column.
  I know I can use <code>.xs()</code> like a following code, when I want to select only one column name and level name.</p>
  
  <pre><code>df.xs('column_name1', level='column_level1', axis=1)
  </code></pre>
  
  <p>In my specific case, I want to select multiple column names like following code.
  (Actually It doesn't work, because .xs doesn't support this way.)</p>
  
  <pre><code>df.xs(['column_name1', 'column_name2'], level='column_level1', axis=1)
  </code></pre>
  
  <p>How to select multiple column names in specific one level?</p>
  
  <hr>
  
  <p>I show more specific code.</p>
  
  <pre><code>import pandas as pd
  import io
  
  data = u"""
  column_name1,column_name2,column_name3
  column_nameA,column_nameB,column_nameC
  0.1,1,10
  0.2,2,20
  0.3,3,30
  """
  df = pd.read_csv(io.StringIO(data), header=[0, 1])
  df.columns.names = ['column_level1', 'column_level2']
  print df
  </code></pre>
  
  <p><code>df</code> is this</p>
  
  <pre><code>column_level1 column_name1 column_name2 column_name3
  column_level2 column_nameA column_nameB column_nameC
  0                      0.1            1           10
  1                      0.2            2           20
  2                      0.3            3           30
  </code></pre>
  
  <p>and, I want to make this data by column name</p>
  
  <pre><code>column_level1 column_name1 column_name2
  column_level2 column_nameA column_nameB
  0                      0.1            1
  1                      0.2            2
  2                      0.3            3
  </code></pre>