title: Calculate Arbitrary Percentile on Pandas GroupBy
body: <p>Currently there is a <code>median</code> method on the Pandas's <code>GroupBy</code> objects.</p>
  
  <p>Is there is a way to calculate an arbitrary <code>percentile</code> (see: <a href="http://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.percentile.html" rel="noreferrer">http://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.percentile.html</a>) on the groupings? </p>
  
  <p>Median would be the calcuation of percentile with <code>q=50</code>.</p>