Stevedore worker plugins

The deferred processor now loads plugins via stevedore, and passes
all event messages to each plugin found. This provides the first
extensibility mechanism for storyboard, by allowing anyone to write
tasks that should be handled asynchronously when the system changes.
Custom workers must implement two methods: 'enabled' and 'handle'.
The first checks to see whether this plugin is enabled, and will be
run only once during initialization. The second handles the event.

The intent is to drive more advanced functionality off of this
mechanism, such as emails, search indexes, and third party tool
integration.

Change-Id: I32b40eab9355c18db1e4ec132b09dc77561a3475