title: List unique values in a Pandas dataframe
body: <p>I know that</p>
  
  <pre><code>df.name.unique()
  </code></pre>
  
  <p>will give unique values in ONE column <code>'name'</code>. </p>
  
  <p>For example: </p>
  
  <pre><code>name    report  year
  Coch    Jason   2012
  Pima    Molly   2012
  Santa   Tina    2013
  Mari    Jake    2014
  Yuma    Amy     2014
  
  array(['Jason', 'Molly', 'Tina', 'Jake', 'Amy'], dtype=object)
  </code></pre>
  
  <p>However, let's say I have ~1000 columns and I want to see all columns' unique values all together.</p>
  
  <p>How do I do it?</p>