title: specific values count in groupby
body: <p>I have the following dataframe:</p>
  <pre><code>df = pd.DataFrame([
      ('A', 'A', 'Cat'),
      ('A', 'A', 'Dog'),
      ('A', 'A', 'Cat'),
      ('A', 'B', 'Dog'),
      ('B', 'B', 'Rat'),
      ('B', 'C', 'Cat'),
      ('B', 'C', 'Cat')
  ], columns=['id', 'group', 'Animal'])
  </code></pre>
  <p>I want to group it by <code>id</code> and <code>group</code> and calculate the occurrence  of <code>Cat</code> in each group. An example output will be:</p>
  <pre><code>[2, 0, 0, 2]
  </code></pre>
  <p>2 <code>cat</code> in group <code>AA</code>,</p>
  <p>0 <code>cat</code> in group <code>AB</code>,</p>
  <p>0 <code>cat</code> in group <code>BB</code>,</p>
  <p>2 <code>cat</code> in group <code>BC</code></p>
  <p>Can anyone help? Thanks!</p>