Implement event_description of EventFormattingAgent.
Omit option details from event_description of WebsiteAgent.
Merge pull request #461 from knu/user-defined-event_description

Improve event_description of EventFormattingAgent and WebsiteAgent.
WebsiteAgent should interpolate values from incoming event payload.
Refactor Drop classes.

Not alone EventDrop but all subclasses now take a hash of "locals" which
could be referenced in interpolation.  This will make it easier for
Agent classes to add custom "locals" for use in interpolation.
Merge branch 'refactor-drop'