Add support for pe_dist_dir to be an HTTP server.

Prior to this change, pe_dist_dir is required to be a local filesystem
path. With this code in place, pe_dist_dir can now be pointed to an
HTTP or HTTPS server, and the LATEST file and PE tarballs will be
properly sourced from that server.
Merge pull request #395 from branan/http_dist_dir

Add support for pe_dist_dir to be an HTTP server.