doc changes

Additions to better explain how to add custom shapefile data as well
as select_ugid.
i218: setup.py does not understand --prefix

Modified setup.py to more tightly integrate with distutils. Custom
commands were implemented as subclass of Command.
i155: seasonal groupings

Added support for temporal aggregation based on arbitrary monthly
groupings. Fixed bug in moving window percentile calculation where
indices were not being correctly mapped.