title: How to find gaps in dates using pandas
body: <p>I have a data set which shows the performance of a device per month. Here is how the data looks like</p>
  
  <pre><code>device, date, performance
     1  , 06/16, 50.4
     1  , 07/16, 54
     1  , 08/16, 6
     1  , 09/17, 67
     1  , 10/17, 56
     2  , 01/12, 34
         ....
  </code></pre>
  
  <p>A device may not be working in some months so there are some gaps in the data. For example, device one has some gaps in the above data. I'd like to create a data frame which shows the time ranges each device was working, here is how the resulting data set should look like</p>
  
  <pre><code>device, start, end
   1    ,  06/16, 08/16
   1    ,  09/17, 10/17
       ....
  </code></pre>
  
  <p>How can I create that using pandas?</p>