title: How to add a column to all DataFrames inside a Panel (don't mind using a for loop)
body: <p>I've tried the code below, but although the column is added, the panel axis doesn't update.
  I'm using Python 2.7.3 and Pandas 0.12.0.</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  
  p = pd.Panel(np.random.rand(2,4,3),
               items=range(2),
               major_axis=pd.date_range('2013/11/10',periods=4),
               minor_axis=map(lambda x: "col%d" % x, range(3)))
  
  p[0]
                  col0      col1      col2
  2013-11-10  0.942665  0.449846  0.194914
  2013-11-11  0.831268  0.031158  0.399326
  2013-11-12  0.079028  0.580820  0.459432
  2013-11-13  0.858297  0.746162  0.457021
  
  p.minor_axis
  Index([u'col0', u'col1', u'col2'], dtype=object)
  
  p[0]['col99'] = 1
  
  p[0]
                  col0      col1      col2  col99
  2013-11-10  0.163177  0.737629  0.784708      1
  2013-11-11  0.269936  0.609961  0.997503      1
  2013-11-12  0.873156  0.295626  0.533119      1
  2013-11-13  0.824352  0.381446  0.613053      1
  
  p.minor_axis
  Index([u'col0', u'col1', u'col2'], dtype=object)
  </code></pre>
  
  <p>The same happens with this code:</p>
  
  <pre><code>p[0].insert(1, 'col99', p[0].col0)
  
  p[0]
                  col0     col99      col1      col2
  2013-11-10  0.942665  0.942665  0.449846  0.194914
  2013-11-11  0.831268  0.831268  0.031158  0.399326
  2013-11-12  0.079028  0.079028  0.580820  0.459432
  2013-11-13  0.858297  0.858297  0.746162  0.457021
  
  p.minor_axis
  Index([u'col0', u'col1', u'col2'], dtype=object)
  </code></pre>
  
  <p>Any ideas?</p>
  
  <p>I think one option would be a command to add another minor_axis. Does this exists?</p>
  
  <p>Thanks,</p>
  
  <p>JM</p>