title: Delete duplicate rows with the same value in all columns in pandas
body: <p>I have a dataframe with about a half a million rows. As I could see, there are plenty of duplicate rows, so how can I drop duplicate rows that have the same value in all of the columns (about 80 columns), not just one?</p>
  
  <p>df: </p>
  
  <pre><code>period_start_time    id    val1    val2    val3
  06.13.2017 22:00:00  i53    32      2       10
  06.13.2017 22:00:00  i32    32      2       10
  06.13.2017 22:00:00  i32    4       2       8
  06.13.2017 22:00:00  i32    4       2       8
  06.13.2017 22:00:00  i32    4       2       8
  06.13.2017 22:00:00  i20    7       7       22
  06.13.2017 22:00:00  i20    7       7       22
  </code></pre>
  
  <p>Desired output: </p>
  
  <pre><code>period_start_time    id    val1    val2    val3
  06.13.2017 22:00:00  i53    32      2       10
  06.13.2017 22:00:00  i32    32      2       10
  06.13.2017 22:00:00  i32    4       2       8
  06.13.2017 22:00:00  i20    7       7       22
  </code></pre>