Improve log summary nagios check significantly

Fixes issue https://github.com/osuosl-cookbooks/rdiff-backup/issues/7

- Check now uses the two most recent job logs instead of just the current one.
- Check now only uses the most recent job run for each job.
- Check now lists failed and missing jobs.
- Check now correctly lists number of jobs, even when jobs are missing or have been run multiple times per day.
- Check now ignores removed jobs.
- Check now notifies of missing or failed purges.
I vote to use ``ls -c`` to be safe. Lets not make too many assumptions about what may or may not happen. Let's worry about the most logical reason.