Add "season" as a time/date component to DatetimeIndex
To help pandas to get better traction in the Earth sciences, it would be awesome to add support for climatological "season" as a [Time/Date Component](http://pandas.pydata.org/pandas-docs/stable/timeseries.html#time-date-components) of `pandas.DatetimeIndex`. Season comes up in geoscience at least as often as "quarter" does in finance.

The standard seasons (the ones worth adding a shortcut for) would be `['DJF', 'MAM', 'JJA', 'SON']`, as labeled by the first letter of each month. These could be represented by integers (in a pinch) but string labels would be much more natural -- people talk about the season "JJA" (or "summer" in the Northern hemisphere), not "season 3".

Actually, this might be a nice show case for the brand new `CategoricalBlock` (#7217).

For reference, here is some vectorized code to do the calculation from month numbers:

``` python
SEASONS = np.array(['DJF', 'MAM', 'JJA', 'SON'])

month = np.arange(12) + 1
season = SEASONS[(month // 3) % 4]
print season 
```

(We actually already use this code to add "season" to [xray](http://github.com/xray/xray), but I think this would be useful more broadly as well.)
