Add options to filter/geoip. closes LOGSTASH-1703

According to the changelog for logstash 1.3.0, the geoip filter has a
"new setting 'add_geojson' (default true) and geojson_name (default
'location') to write a GeoJSON-friendly coordinate to your event".
However, these options were never implemented and the fields were
hardcoded in 'lib/logstash/filters/geoip.rb'. This commit fixed this
oversight and adds tests for these options.