Create multiple cron jobs in a single cron.d file

I've added cron::multiple and cron::multiple::job. The idea is to let the user pass a hash of jobs to create multiple jobs within a single file. It's purely a cosmetic/housekeeping change as each job still requires its own unique name, but it does make working with large numbers of cron jobs easier.

I haven't written spec tests for the new classes as I don't expect this PR to be accepted as is. Looking more for input and discussion at first. What would be the best way to implement this?