Only accept a new master serial if the site's serial is also new

This check complements the existing one that is based solely on the
site->date < master->date -> syncing
assumption, but is not affected by site serial bumps without a new
master serial.

Closes #9 without requiring modifications to the sync scripts.
Include the web-demo header in the noscript comment in the demo
Limit the number of different mirrors to choose from

With this change even if the subset of mirrors whose distance <= std-dev
consists of more than four mirrors, those mirrors whose distance is
greater than the fourth (sorted by distance) won't be in the final
subset.

Closes #2 at least for the time being
Merge branch 'issue-2-ng'
Merge branch 'issue-9'

I don't like to just add more and more fields to the database, but I
think that fixing bugs is more important. The model can be improved
later on, probably with the help of a much-needed abstraction of the
database.