Add secondary (remote) repository storage for mirroring

[ArneBachmann]

This is another variation of the dangerous "we become a full-fledged (D)VCS" theme. It is, however, the most practical solution so far, as it allows to achieve several goals at the same time:

• Immediate offsite backup
• Allows shared (read) usage (e.g. introduce read-only flag)
• Avoids full repo dumps and rsync scripts