implement Job#tags and Scheduler#jobs(:tag[s] => x)
implement Scheduer#running_jobs(:tag/:tags => x)
spec AtJob#time
parse points in time with named timezones
drop rufus/scheduler/timezone.rb
spec Job.new(:tag / :tags)
spec Scheduler#unschedule(job_or_job_id)
spec Scheduler#cron_jobs
document Job methods