title: pandas get rows which are NOT in other dataframe
body: <p>I've two pandas data frames that have some rows in common.</p>
  <p>Suppose dataframe2 is a subset of dataframe1.</p>
  <p><strong>How can I get the rows of dataframe1 which are not in dataframe2?</strong></p>
  <pre><code>df1 = pandas.DataFrame(data = {'col1' : [1, 2, 3, 4, 5], 'col2' : [10, 11, 12, 13, 14]}) 
  df2 = pandas.DataFrame(data = {'col1' : [1, 2, 3], 'col2' : [10, 11, 12]})
  </code></pre>
  <p>df1</p>
  <pre><code>   col1  col2
  0     1    10
  1     2    11
  2     3    12
  3     4    13
  4     5    14
  </code></pre>
  <p>df2</p>
  <pre><code>   col1  col2
  0     1    10
  1     2    11
  2     3    12
  </code></pre>
  <p>Expected result:</p>
  <pre><code>   col1  col2
  3     4    13
  4     5    14
  </code></pre>