title: Creating a pandas DataFrame from columns of other DataFrames with similar indexes
body: <p>I have 2 DataFrames df1 and df2 with the same column names ['a','b','c'] and indexed by dates.
  The date index can have similar values.
  I would like to create a DataFrame df3 with only the data from columns ['c'] renamed respectively 'df1' and 'df2' and with the correct date index. My problem is that I cannot get how to merge the index properly.</p>
  
  <pre><code>df1 = pd.DataFrame(np.random.randn(5,3), index=pd.date_range('01/02/2014',periods=5,freq='D'), columns=['a','b','c'] )
  df2 = pd.DataFrame(np.random.randn(8,3), index=pd.date_range('01/01/2014',periods=8,freq='D'), columns=['a','b','c'] )
  df1
                   a        b            c
  2014-01-02   0.580550    0.480814    1.135899
  2014-01-03  -1.961033    0.546013    1.093204
  2014-01-04   2.063441   -0.627297    2.035373
  2014-01-05   0.319570    0.058588    0.350060
  2014-01-06   1.318068   -0.802209   -0.939962
  
  df2
                   a        b            c
  2014-01-01   0.772482    0.899337    0.808630
  2014-01-02   0.518431   -1.582113    0.323425
  2014-01-03   0.112109    1.056705   -1.355067
  2014-01-04   0.767257   -2.311014    0.340701
  2014-01-05   0.794281   -1.954858    0.200922
  2014-01-06   0.156088    0.718658   -1.030077
  2014-01-07   1.621059    0.106656   -0.472080
  2014-01-08  -2.061138   -2.023157    0.257151
  </code></pre>
  
  <p>The df3 DataFrame should have the following form : </p>
  
  <pre><code>df3
                   df1        df2
  2014-01-01   NaN        0.808630
  2014-01-02   1.135899   0.323425
  2014-01-03   1.093204   -1.355067
  2014-01-04   2.035373   0.340701
  2014-01-05   0.350060   0.200922
  2014-01-06   -0.939962  -1.030077
  2014-01-07   NaN        -0.472080
  2014-01-08   NaN        0.257151
  </code></pre>
  
  <p>But with NaN in the df1 column as the date index of df2 is wider. (In this example, I would get NaN for the ollowing dates : <code>2014-01-01, 2014-01-07 and 2014-01-08</code>)</p>
  
  <p>Thanks for your help.</p>