title: Pandas: select the first couple of rows in each group
body: <p>I can't solve this simple problem and I'm asking for help here...
  I have DataFrame as follows and I want to select the first two rows in each group of 'a'</p>
  
  <pre><code>df = pd.DataFrame({'a':pd.Series(['NewYork','NewYork','NewYork','Washington','Washington','Texas','Texas','Texas','Texas']), 'b': np.arange(9)})
  
  df
  Out[152]: 
              a  b
  0     NewYork  0
  1     NewYork  1
  2     NewYork  2
  3  Washington  3
  4  Washington  4
  5       Texas  5
  6       Texas  6
  7       Texas  7
  8       Texas  8
  </code></pre>
  
  <p>that is, I want an output as follows:</p>
  
  <pre><code>            a  b
  0     NewYork  0
  1     NewYork  1
  2  Washington  3
  3  Washington  4
  4       Texas  5
  5       Texas  6
  </code></pre>
  
  <p>Thanks a lot for the help.</p>