title: Identify records NOT in another dataframe
body: <p>I have one dataframe like this:</p>
  
  <pre><code>data1 = pd.DataFrame([['a','z',0],['a','y',20],['b','z',1]],columns=['id1','id2','number'])
  data2 = pd.DataFrame([['a','y',1],['a','y',1],['b','z',0]],columns=['id1','id2','number'])
  </code></pre>
  
  <p>I want to return the records that are in data1 and not data2 (as joined by id1 and id2).</p>
  
  <p>In this case, I would just want it to return one record ['a','z',0] since both ['a','y'] and ['b','z'] do exist in data2.</p>