implemented crude curl for async data read from mapzen vector tile service for geojson tiles. Code cleaning required, and updateTile function should be driven by number of tiles to be read having tileCoordinates calculated based on view.
Adding Async libcurl code. Also made tileManager govern what tile coordinates to be loaded, which is passed to the dataSource to be fetched from the vectortile service using libcurl multi interface
Merge branch 'dev' of https://github.com/tangram-map/tangram-es into dev
some basic commenting