Get the Daily Snapshot working again

The Daily Snapshot stopped working when we moved to git and GitHub. This gets
it working again and updates the installation instructions/backup script.

Note that the new --copy-dhroot option in dh-newuser will only work properly
for the Daily Snapshot as of right now; you can't use it to save cloning from
arbitrary Dreamwidth forks. Its purpose right now is simply to prevent needless
calls to GitHub, since this would be happening every day.

With Mark's permission, I've also set the Daily Snapshot email address back to
my own.
Add update-skel to INSTALL and clean up

I forgot to add the update-skel command to the install instructions in the last
commit. I also generally tidied up the instruction added in the last commit so
that it's in a style consistent with the other instructions.
Add -l switch to su and indent it for consistency

Oops, noticed this wasn't indented, and I also missed a -l switch.
Silence the getent call

We don't need to have dh-daily's /etc/passwd entry show up in the cron email.
Merge pull request #20 from Sophira/daily-snapshot

Daily Snapshot