Refactoring - renamed in Engine `close_if_idle` to `stop_if_idle`.
When `spider_idle` event happens and more requests are scheduled, engine
is not stopped even though DonStopEngine wasn't raised.
Added `command_invoked` field to Engine.
Added utils/sitemap module.
Added `SitemapSpider`.
Added `req_or_resp` artument for Settings' get() methods.
Applied settings with `req_or_resp` to HttpDownloadHandler.
Applied per request settings to `Filter` middleware.
Fixed conditions on non-200 statuses.
Raise `FilterError` in Filter mw, when filter is applied.
Refactoring - renamed error `RequestDropped` to `DropRequest`.
Fixed `Request.details` error.
Refactoring - in `RestartPipeline` exception, renamed `new_value` with
`new_request`.
Removed `engine_stopping` signal.
Fixed engine stopping. Better put it in a separate deferred in a case `engine.stop()` is called directly from spider or some mw.