title: Formatting datetime in pandas columns as Quarters
body: <p>I have a <code>DataFrame</code> with columns in <code>DateTime</code> index, representing quarters such as: <code>2000-03-31 00:00:00</code></p>
  <p>How can I do to transform this into <code>'2000q1'</code>?</p>
  <p>I have looked around the docs, but they only mention <a href="http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DatetimeIndex.quarter.html" rel="nofollow noreferrer"><code>DateTimeIndex.quarter</code></a></p>
  <p><code>format='%Y%q'</code> does not work. Neither does the option <code>on='%Y%q</code></p>