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.
How would one trigger an error if the modification time was incorrect?  Wouldn't it have to verify it against the date in the filename?  At that point, I don't see why you wouldn't just use the filename directly.  I suppose the script could be modified to look for log files specifically from the current and previous day (or previous and even more previous day in case the log for the current day has not been created yet), but it seems like more of a hassle than it's worth.  Why would people be dropping random files into the log dir or renaming the logs anyway?  I feel like it's pretty obvious that messing with random files on a filesystem will cause bad things to happen.