Add Agent#short_type.
Fix a leftover of JsonPath to Liquid conversion.
EventFormattingAgent: Enable access to upstream agent info via "agent".

With this enhancement you can, for example, embed the site URL in an
event created by WebsiteAgent by interpolating like
"{{agent.options.url}}", or add a message prefix depending on the type
of upstream agent using the "case" construct of Liquid.
Remove the "skip_agent" option from EventFormattingAgent.

It is now possible to include an agent name without the special flag
just by putting `"agent" => "{{agent.type}}"` in "instructions".

This commit includes automatic migration.
Fix specs after the transition from skip_agent to {{agent}}.
Add specs for AgentDrop.
Fix grammar.
Make {{agent}} universally accessible from any agent.

To implement this, EventDrop is introduced to represent an Event object
in interpolation.
Add a hover-help for the Options field which explains interpolation.
EventFormattingAgent: Partial revert: Do not include "agent" to payload.
Introduce a new concern LiquidDroppable for refactoring.
Merge pull request #410 from knu/provide_access_to_agent

EventFormattingAgent: Provide access to upstream agent