[WIP] Foxx queues

* [x] Implement queues API
* [x] Implement jobs API
* [x] Run Foxx.queues.manager.run after Foxx initialization in exactly one thread (@fceller)
* [x] Run Foxx initialization on startup instead of on demand (@fceller)
* [x] Implement Foxx.queues.manager.run
* [x] Implement Foxx.queues.manager.manage
* [x] Implement Foxx.queues.worker.work
* [x] Allow scheduled jobs
* [x] Run jobs in predictable order
* [x] Implement onSuccess/onFailure handlers for jobs
* [x] Add retry delay (exponential back-off) for jobs with maxFailures > 0
* [x] Log failures in job document
* [ ] Tests? (at least acceptance tests)
* [ ] Add hash indexes for the new system collections?
* [x] Documentation
* Reference implementations (mailer for major transactional e-mail services):
  * [x] Mailgun (https://github.com/arangodb/foxx-mailer-mailgun)
  * ~~Mandrill (https://github.com/arangodb/foxx-mailer-mandrill)~~ (too complex)
  * [x] PostageApp (https://github.com/arangodb/foxx-mailer-postageapp)
  * [x] Postmark (https://github.com/arangodb/foxx-mailer-postmark)
  * [x] SendGrid (https://github.com/arangodb/foxx-mailer-sendgrid) (@moonglum: contact re/ dev account)

@fceller just make sure `require('org/arangodb/foxx/queues/manager').run();` is called once Foxx is initialised. It takes care of creating the periodic task (invoking `manage()`) by itself.