Provide access to schedule to allow spec phase tests

[hlascelles]

This allows programmatic access to the loaded and parsed schedule, which will allow a spec phase check of the configuration.

This will perform a variety of sanity checks and ensure that:

1. The yml is present and valid
2. The job classes exist and are descendants of Que::Job
3. The cron fields are present and valid
4. The queues (if present) are strings
5. The priorities (if present) are integers
6. The schedule_types are known

  describe 'check que_schedule.yml' do
   it 'loads the schedule from the default location' do
     expect(Que::Scheduler.schedule).not_to be nil
   end
 end

[fameoflight]

LGTM 💯