title: Pandas: return number of occurrences by date
body: <p>Assume I have the following DataFrame</p>
  
  <pre><code>dic = {"Date": ["04-Jan-16", "04-Jan-16", "04-Jan-16", "05-Jan-16", "05-Jan-16"], 
         "Col": ['A', 'A', 'B', 'A', 'B']}
  
   df = pd.DataFrame(dic)
  
   df
  
    Col       Date
  0   A  04-Jan-16
  1   A  04-Jan-16
  2   B  04-Jan-16
  3   A  05-Jan-16
  4   B  05-Jan-16
  </code></pre>
  
  <p>I need to return a second DataFrame that lists the number of occurrences of A and B by day. i.e.</p>
  
  <pre><code>           A  B
  Date           
  04-Jan-16  2  1
  05-Jan-16  1  1
  </code></pre>
  
  <p>I have a feeling this involves "groupby", but I dont know enough about it to get it into the format above ^</p>