rewrite of UDD importers runner

[Lucas Nussbaum <lucas@debian.org>]

Hi,

I changed the way UDD importers are executed.

Previously, they were executed via cron.

Now, there's a small cron-like daemon taking care of executing them.
- the importers are grouped into 'pools'. only one importer of each pool
  can run at a given time, which should help reduce the load on the server
  at some times.
- the exit status and the output of the execution is stored in a db.
- there's a status page showing the status of each importer:
  https://udd.debian.org/udd-status.cgi
- since the importers no longer run from cron, no mails are sent when
  they fail. Instead, there's a separate script providing a list of
  current errors via email, ran 4 times a day via cron. Please don't
  ignore this email, but help fix the errors instead. (I'm working on the
  ones for ubuntu-bugs and archive-ubuntu.)
- to add an importer, simply add it to rudd.conf.
- all importers have been migrated. There's still a few other things
  running via cron (database dumps, for example).

Please let me know if you have questions / want more info.

Lucas