title: Concatenate rows in python dataframe
body: <p>This question may be very basic, but I would like to concatenate three columns in a pandas DataFrame.<br>
  I would like to concatenate col1, col2 and col3 into col4.  I know in R this could be done with the paste function quite easily.</p>
  
  <pre><code>df = pd.DataFrame({'col1': [2012, 2013, 2014], 'col2': 'q', 'col3': range(3)})
  </code></pre>
  
  <p>Edit: Code for clarity - I would like to generate col4 automatically:</p>
  
  <pre><code>x=pd.DataFrame()
  x['col1'] = [2012,2013,2013]
  x['col2'] = ['q', 'q', 'q']
  x['col3'] = [1,2,3]
  x['col4'] = ['2012q1', '2013q2', '2014q4']
  </code></pre>