title: Count unique values per groups with Pandas
body: <p>I need to count unique <code>ID</code> values in every <code>domain</code>.</p>
  <p>I have data:</p>
  <pre><code>ID, domain
  123, 'vk.com'
  123, 'vk.com'
  123, 'twitter.com'
  456, 'vk.com'
  456, 'facebook.com'
  456, 'vk.com'
  456, 'google.com'
  789, 'twitter.com'
  789, 'vk.com'
  </code></pre>
  <p>I try <code>df.groupby(['domain', 'ID']).count()</code></p>
  <p>But I want to get</p>
  <pre><code>domain, count
  vk.com   3
  twitter.com   2
  facebook.com   1
  google.com   1
  </code></pre>