aurweb: Mirror aur.git to GitHub[1]

This is meant as a extra "backup" and as another way for our users to fetch PKGBUILDs from the AUR. It also allows the community to create their own (perhaps better) "AUR" API/database as all essential data is now available (this + [2]).

At the monent this is experimental and we aren't committing to keeping it around.

[1] https://github.com/archlinux/aur [2] http://aur.archlinux.org/packages-meta-ext-v1.json.gz