Remove "locals" from Drop classes and introduce a real context class.

The Context class is capable of switching a self object and adding local
variables, and also hashable for the convenience of being used as a
cache key.

As for the thread safety, we give it up for now as we don't support it
or have concurrency tests yet anyway.
WebsiteAgent: Provide a variable _response_ for interpolation.

* `_response_.headers.Header-Name` is expanded to the value of header
  named _Header-Name_.

* `_response_.status` is expanded to the HTTP status as integer, which
  is almost always 200, since currently the WebsiteAgent will only
  create an event when a request succeeds, and the request method is
  limited to GET.
Add specs for `interpolation_context#[]=` and `interpolate_with`.
Merge pull request #466 from knu/website_agent-response_drop

Give access to response status and header values in WebsiteAgent.

Closes #445.