Allow null geometry objects. Fixes #18.
Use spherical area for simplification.
Oops, fix conversion between absolute & relative.
Use absolute area for simplification.
Port d3.geo.area to compute triangle area.
Fix for counterclockwise triangles.
Fix (again) for spherical triangle area.
Document units for simplification.
Slight optimisation.
Merge branch 'simplify-spherical-area' of https://github.com/jasondavies/topojson into simplify-spherical-area
Merge branch 'simplify-spherical-area'
Update version number.
Optimise and fix triangle area calculation.

When investigating possible optimisations, I realised that I had made an
error in my original implementation using l’Huillier.  The error was
that s = a + b + c should have been s = (a + b + c) / 2.  The s stands
for semiperimeter!

As a result, the areas were slightly inflated, but not catastrophically
enough for us to notice by eye.

This fix is an optimised implementation using a formula due to van
Oosterom and Strackee (1983).
Rename us.json to us-10m.json.
Bump version number.