Improve area accuracy for small polygons.
Improve area accuracy for small polygons.

The use of cosines can introduce large rounding errors for small
distances.

Fixes #28.

tsl3su/WorldMap_Choropleth,studiowangfei/topojson,youprofit/topojson,lyhpcha/topojson,whoinlee/topojson,mbostock/topojson,hcxiong/topojson,btahir/topojson,ststeiger/topojson,AlpinHologramm/topojson,koshlan/topojson,ongerit/topojson,maxogden/topojson