HTTP/HTTPS vector tile sources

This builds on @rclark's patches in #61--248beafb46496d68085dfd219bc095034f367a50 prevents TM2 from treating http(s) sources as local and trying to create (invalid) directories for them.

I've been using this successfully with [mojodna/tiled-vector-remix](https://github.com/mojodna/tiled-vector-remix).