Add in additional manifest to setup postgres backups

- Ensures that a directory *on the same box* is created
- Sets a job to run every 2 hours using pg_dump
- Uses a template I found on the www's: https://github.com/ckaenzig/puppet-postgresql/blob/master/templates/pgsql-backup.sh.erb
Merge pull request #248 from alphagov/postgres_backups

Add in additional manifest to setup postgres backups