Improve performance

John,

This change creates two new shapefiles in spherical mercator projection. They should render much faster since they will not longer need to be reprojected on the fly during rendering (which can be quite slow unless you are running a mapnik version with [this fix](https://github.com/mapnik/mapnik/issues/1703)).

Also I created spatial indexes for all the shapefiles using Mapnik's `shpindex` tool. This will speed up rendering at high zoom levels by making the queries faster for the select set of points within a given tile bounds.

I think this should fix the issues you've been seeing as per http://support.mapbox.com/discussions/tilemill/5301-tilemill-running-on-aws-error-message-sigkill, no need to fuss with memory settings.

Dane