title: Unique values of two columns for pandas dataframe
body: <p>Suppose I have pandas data frame with 2 columns:</p>
  
  <pre><code>df: Col1  Col2
        1     1
        1     2
        1     2
        1     2
        3     4
        3     4
  </code></pre>
  
  <p>Then I want to keep only the unique couple values (col1, col2) of these two columns and give their frequncy:</p>
  
  <pre><code>df2: Col1  Col2  Freq
        1     1     1
        1     2     3
        3     4     2
  </code></pre>
  
  <p>I think to use <code>df['Col1', 'Col2'].value_counts()</code> but it works only for one column. 
  Does it exist a function to deal with many columns?</p>