risk/writers:

Added LossMapGeoJSONWriter, which is basically the GeoJSON equivalent
of the LossMapXMLWriter.

I also abstracted a base LossMapWriter class to share code common to
all such writers.
tests/risk/writers_test:

Added tests for LossMapGeoJSONWriter. Also some minor tweaks to the
LossMapXMLWriter tests.
risk/writers:

pep8 whitespace cleanup
risk/writers:

Updated the LossMapGeoJSONWriter to use a slightly cleaner way of
serializing to a file.
Merge pull request #99 from larsbutler/geojson-lossmap-writer

GeoJSON LossMap Writer

[r=micheles] [f=1198881]