a backup script to send backups over email when changed

[gasche]

This is a home-grown backup script that I built for a local HotCRP
installation, but I thought that it may be useful to others as
well. The backup strategy is detailed as follows:

• a backups/ directory at the project root with at most two backups saved,
  the last one (new) and the one before that (old)

• whenever the script runs, it moves new to old and creates a new
  new file using the backupdb.sh script; then it compares new
  and old, and sends the new backup by email if it differs from the
  old one

• if a passphrase file exists in the backups directory, then
  it is used as a passphrase for gpg to encrypt the backup file

Implementation remarks:

• we are careful to pass --skip-dump-date to mysqldump, otherwise the backups would always differ
• we are careful to cd to its base directory, so that the script can be invoked directly from a crontab.