title: python/pandas - converting date and hour integers to datetime
body: <p>I have a dataframe that has a date column and an hour column.        </p>
  
  <pre><code>     DATE         HOUR
    2015-1-1         1
    2015-1-1         2
        .            .
        .            .
        .            .
    2015-1-1         24
  </code></pre>
  
  <p>I want to convert these columns into a datetime format something like:
  <code>2015-12-26 01:00:00</code>  </p>