added gem: execjs
added ExecJS; added single-file topojson library; evaluates geojson to topojson in TopojsonHelper and returns result
fix for CORS; added 'Access-Control-Allow-Origin' header
when geojson created record.dm is set to "id" => when topojson created property does not need promotion
Merge pull request #7 from radcliff/execjs

Execjs