title: Expand pandas dataframe column of dict into dataframe columns
body: <p>I have a Pandas DataFrame where one column is a Series of dicts, like this:</p>
  
  <pre><code>   colA  colB                                  colC
  0     7     7  {'foo': 185, 'bar': 182, 'baz': 148}
  1     2     8  {'foo': 117, 'bar': 103, 'baz': 155}
  2     5    10  {'foo': 165, 'bar': 184, 'baz': 170}
  3     3     2  {'foo': 121, 'bar': 151, 'baz': 187}
  4     5     5  {'foo': 137, 'bar': 199, 'baz': 108}
  </code></pre>
  
  <p>I want the <code>foo</code>, <code>bar</code> and <code>baz</code> key-value pairs from the dicts to be columns in my dataframe, such that I end up with this:</p>
  
  <pre><code>   colA  colB  foo  bar  baz
  0     7     7  185  182  148
  1     2     8  117  103  155
  2     5    10  165  184  170
  3     3     2  121  151  187
  4     5     5  137  199  108
  </code></pre>
  
  <p>How do I do that?</p>