Read SRTM files.

Very, very much a work in progress.

1. Go download an SRTM hgt file. example:
   wget http://dds.cr.usgs.gov/srtm/version2_1/SRTM1/Region_01/N45W122.hgt.zip && unzip N45W122.hgt.zip
2. Turn it into some json with node index.js. You'll see heights.json in your directory.