Plugins may now register cron workers.

This adds a crontab plugin hook to StoryBoard, allowing a plugin
developer to run periodic events. Example use cases include:
- Summary emails.
- Periodic report generation.
- Synchronization check points.

Plugins are expected to provide their own execution interval and
configuration indicator. The management of cron workers is
implemented as its own cron plugin as a sample, and unit tests
for all components are provided.

Change-Id: I3aa466e183f1faede9493123510ee11feb55e7aa
Merge "Plugins may now register cron workers."