support repo mirroring

There are more and more reasons to support mirroring of the official f-droid.org archive:

• automatic mirror selection in the client (fdroidclient#35)
• improve our "bus factor", reducing reliance on things run by a single person
• the U Twente people who host Debian infrastructure offered to mirror us

This could be handled in many ways. Right now, its possible to hack together a mirror by downloading the index.xml, then just downloading all of the files referenced in there. That would keep things on the f-droid.org server side nice and simple: no new daemons needed.

It would be much nicer IMHO to allow rsync mirroring. Its reliable, available everywhere, and easy to setup. We could start with an rsyncd on f-droid.org that has an IP whitelist for a set of "official" mirrors.