title: Combine date column and time column into datetime
body: <p>I have two columns (both text objects), one date, the other hour-ending.</p>
  
  <pre><code>df = pd.DataFrame({'Date' : ['2018-10-01', '2018-10-01', '2018-10-01'],
                  'Hour_Ending': ['1.0', '2.0', '3.0']})
  </code></pre>
  
  <p>How do I add the two columns together to get a datetime object that looks like this?</p>
  
  <p>2018-10-01 01:00</p>
  
  <p>As a bonus, how do I change Hour_Ending to Hour_Starting?</p>