title: long-format pandas dataframe to dictionary
body: <p>While I find help and documentation on how to convert a pandas DataFrame to dictionary so that columns are keys and values are rows, I find myself stuck when I would like to have one of the column's values as keys and the associated values from another column as values, so that a df like this</p>
  
  <pre><code>a b
  1 car
  1 train
  2 boot
  2 computer
  2 lipstick
  </code></pre>
  
  <p>converts to the following dictionary <code>{'1': ['car','train'], '2': ['boot','computer','lipstick]}</code></p>
  
  <p>I have a feeling it's something pretty simple but I'm out of ideas. I tried <code>df.groupby('a').to_dict()</code> but was unsuccessful</p>
  
  <p>Any suggestions?</p>