hazard/writers:

Factored out HazardMapWriter base class from HazardMapXMLWriter.
hazard/writers:

Added HazardMapGeoJSONWriter class.
tests/hazard/writers_test:

Added tests for HazardMapGeoJSONWriter.
hazard/writers:

When building the metadata dict for hazard map geojson outputs,
don't include attributes which are None/irrelevant.
Merge pull request #100 from larsbutler/geojson-hazmap-writer

Hazard map GeoJSON writer

[r=micheles] [f=1199655]