add GeoJSON track format

Switching to GeoJSON as response format for brouter-web allows native parsing and to get rid of the GPX plugin as GeoJSON is the standard Leaflet format. BRouter calculated track stats are passed and displayed.

This is part of an alpha3 release that is available in brouter-web master and ready for deployment. Depending on when you choose to deploy, I might continue adding stuff that has no depencencies, for which it doesn't matter if gets included or not.