Leaflet features using datasource

Got rid of leaflet specific feature support in GeoDataCollection
Made a 2D visualizer based on leaflet for points, polylines, polygons, billboards and labels.
Leaflet now supports czml and kml via cesium datasources.
Switched to leaflet popup for all feature and wms click info popups
Added timeline to 2D mode
I think it would be better to update our `.jshintrc` file (under `src`) rather than adding this here.