Add cron style events

[doughsay]

• Each object should have a crontab.
• The crontab is a list of jobs.
• A job is a cron-spec (e.g. 0 30 11 * * 1-5), a verb name, and a boolean (enabled).
• e.g. {spec: '* * * * * *', verb: 'think', enabled: true}
• This is registered by the server in a global cron registry, and executed at the appropriate times.