Schedule jobs using cron expressions

[jefmathiot]

Add support for Cron syntax to help people coming from Cron schedules to make the migration seamlessly:

job "I-love-my-job" do
  schedule "cron", expression:  "0 1,7,13,20 * * *"
end

See #7.

[jefmathiot]

Documentation here http://www.electricsheep.io/v0.5.0/docs/schedule-your-backups#section-cron

[duckmole]

Close after review