title: Access dictionary keys and values in pandas dataframe column
body: <p>I've got a simple dataframe with a column populated by a python dictionary, in the form:</p>
  
  <pre><code>User                          CLang
  111                      {u'en': 1}
  112            {u'en': 1, u'es': 1}
  112            {u'en': 1, u'es': 1}
  113  {u'zh': 1, u'ja': 1, u'es': 2}
  113  {u'zh': 1, u'ja': 1, u'es': 2}
  113  {u'zh': 1, u'ja': 1, u'es': 2}
  114                      {u'es': 1}
  113  {u'zh': 1, u'ja': 1, u'es': 2}
  </code></pre>
  
  <p>The <code>CLang</code> column contains the frequency of different values for each user. How may I have access to single keys and values of the <code>CLang</code> column? For instance I would like to groupby the <code>User</code> and the most frequent value inside the dictionary, in a form like:</p>
  
  <pre><code>g = df.groupby(['User','CLang')
  </code></pre>
  
  <p>counting then the number of occurrences for each value:</p>
  
  <pre><code>d = g.size().unstack().fillna(0)
  </code></pre>
  
  <p>The resulting dataframe would appear as:</p>
  
  <pre><code>DLang  en  es
  User                 
  111     1   0
  112     1   1
  113     0   4
  114     0   1
  </code></pre>