title: Create Datetime from year and month hidden in multi-index
body: <p>I have a dataframe where year and month are hidden in the <code>multi-index</code>. I want to create a datetime index as additional column (or separate series with same index).</p>
  
  <pre><code>                                           price            
                                              mean     mom_2  
  foo          bar              year month                                      
  997182819645 11               2010 1      1.1900  3.000000  
                                     2      2.2625  4.001769  
  </code></pre>
  
  <p>I thought of adding the two levels of indices together as strings, and then read in that sequence into <code>pd.to_datetime()</code>. However, adding the two indices, I faced problems. I can add them up as integers just fine, but if I want to add them up as strings, I face some error:</p>
  
  <pre><code>In[193]: df.index.get_level_values('year').values.astype(str)
  Out[193]: 
  
  array(['2010', '2010', '2010', ..., '2014', '2014', '2014'], 
        dtype='&lt;U21')
  In[194]: df.index.get_level_values('month').values.astype(str)
  Out[194]: 
  
  array(['1', '2', '3', ..., '10', '11', '12'], 
        dtype='&lt;U21')
  In[195]: df.index.get_level_values('month').values.astype(str) + df.index.get_level_values('year').values.astype(str)
  
  TypeError: ufunc 'add' did not contain a loop with signature matching types
   dtype('&lt;U21') dtype('&lt;U21') dtype('&lt;U21')
  </code></pre>
  
  <p>How can I add create the datetime index here?</p>