title: Pandas cumulative count
body: <p>I have a data frame like this:</p>
  <pre class="lang-none prettyprint-override"><code>0        04:10  obj1
  1        04:10  obj1
  2        04:11  obj1
  3        04:12  obj2
  4        04:12  obj2
  5        04:12  obj1
  6        04:13  obj2
  </code></pre>
  <p>Wanted to get a cumulative count for all the objects like this:</p>
  <pre class="lang-none prettyprint-override"><code>idx      time   object   obj1_count   obj2_count 
  0        04:10  obj1        1             0
  1        04:10  obj1        2             0
  2        04:11  obj1        3             0
  3        04:12  obj2        3             1
  4        04:12  obj2        3             2
  5        04:12  obj1        4             2
  6        04:13  obj2        4             3
  </code></pre>
  <p>Tried playing with cumsum but not sure that is the right way. Any suggestions?</p>