title: Expand pandas DataFrame column into multiple rows
body: <p>If I have a <code>DataFrame</code> such that:</p>
  
  <pre><code>pd.DataFrame( {"name" : "John", 
                 "days" : [[1, 3, 5, 7]]
                })
  </code></pre>
  
  <p>gives this structure:</p>
  
  <pre><code>           days  name
  0  [1, 3, 5, 7]  John
  </code></pre>
  
  <p>How do expand it to the following?</p>
  
  <pre><code>   days  name
  0     1  John
  1     3  John
  2     5  John
  3     7  John
  </code></pre>