title: Find a shared time (overlap) in time columns in python
body: <p>I'm trying to find a shared time where two datetime columns representing a start and an end time overlap with other records.</p>
  
  <p>for example if we have these two columns:</p>
  
  <pre><code>Start                    End 
  2016-08-22 20:20:00      2016-08-22 20:30:00   
  2016-08-22 20:55:00      2016-08-22 21:53:00   
  2016-08-22 21:38:00      2016-08-22 21:58:00
  </code></pre>
  
  <p>I want to check the overlap between them, the output would be:</p>
  
  <pre><code> Start                    End                   Overlap
  2016-08-22 20:20:00      2016-08-22 20:30:00    NaN
  2016-08-22 20:55:00      2016-08-22 21:53:00   2016-08-22 21:38:00
  2016-08-22 21:38:00      2016-08-22 21:58:00   2016-08-22 21:38:00
  </code></pre>
  
  <p>is there an efficient way to achieve it? </p>