Install:

  * ttdnsd from their unstable repository (it's not distributed through the
    Wheezy one)
  * Tor from their Wheezy repository (with higher priority than the official
    Debian repositories, to match what we are doing in the Squeeze images)
  * other packages from their Wheezy repository, at APT's liking (pinned to 990,
    like the official Wheezy and wheezy-updates repositories)