Changes for backup script

Since we now have proper install instructions, here's the promised changes to the backup script to make it a bit more sensible.

The backup script still backs up the /dreamhack directory because it's small enough that we might as well, but it's strongly recommended to use the Git repository instead. (The backup script doesn't include the Git repository directory, but does include instructions on how to recreate it from the online repo if for some reason you *really* want to use the backup.)

A few things are no longer backed up by the script as they're included in the /setup directory instead. The database is still fully backed up, as are the Perlbal site configs, and the generated lists of installed Ubuntu packages and Perl distributions. Other than that, though, everything else (other than the /dreamhack directory backup, as mentioned before) has been removed from the backup script, and the README file has been completely rewritten.