Add cron tasks to manage audit_atom table  #424

Resolves #424
Added configurable options to dump daily audit reports and prune older logs from the table with or without dumping them to a file first. Added rotate method to Ilios_Logger which can be used to rotate and compress log files

**Things to take a look at before merging:**

1. default.ilios.php - I followed the patterns already in this file, including default absolute log files paths in the /web/ilios/cron directory.  Is that correct?  I don't know anything about how this file is implemented.

2. I formatted the audit logs in english with the words at update/create/delete, column, on row of the table.  These aren't user facing strings, should the still go through translation?

3. I wrote a phpunit test for the Ilios_Logger::rotate(), but I couldn't find any infrastructure for testing controllers like the cron controller and didn't want to start from scratch.  Did I miss anything there?