* Nix can now fetch prebuilts (and other files) from the network, iff

  a mapping from the hash to a url has been registered through `nix
  regurl'.

* Bug fix in nix: don't pollute stdout when running tar, it made
  nix-switch barf.

* Bug fix in nix-push-prebuilts: don't create a subdirectory on the
  target when rsync'ing.