title: Group pandas dataFrame by a user defined span of months
body: <p>What is the best approach to group data into winter seasons starting from Oktober to April ? With the evenly distributed frequencies of <a href="http://pandas.pydata.org/pandas-docs/version/0.17.0/generated/pandas.Grouper.html" rel="nofollow noreferrer"><code>TimeGrouper</code></a> I don't get it to output the seasonal sum of wintermonths from seasons 1972/1973, 1973/1974 etc... Maybe a trivial thing, but I don't know how to do this without starting to write a overkill solution.                          </p>
  
  <pre><code>                 sd_x       sd_y
  1972-10-31   0.000000   0.709677
  1972-11-30   1.720838   4.366667
  1972-12-31  15.893438   5.600000
  1973-01-31   6.256230   6.548387
  1973-02-28   0.653714  53.142857
  1973-03-31   0.000000  70.354839
  1973-04-30   0.000000  11.700000
  1973-10-31   0.000000   0.096774
  1973-11-30   0.000000   4.266667
  1973-12-31   0.394652  53.419355
  1974-01-31   4.540915  46.645161
  1974-02-28   2.978056  35.571429
  1974-03-31   0.000000   4.967742
  1974-04-30   0.000000   0.000000
  1974-10-31   0.000000   0.064516
  1974-11-30   0.000000   1.000000
  1974-12-31   5.585954  20.096774
  1975-01-31  50.498147  24.580645
  1975-02-28  35.906097  22.000000
  1975-03-31   0.457109   5.483871
  1975-04-30   0.000000   0.433333
  </code></pre>