Added Redirect pipeline middleware.
Added string representation of Headers.
Added `request_http_repr()` method.
Added `response_http_repr()` method.
Use Twisted RESPONSES to find out the string repr of response status.
It is more complete.
Added `reason` parameter for signals `engine_stopping` and `engine_stopped`.
Use `engine.close_if_idle` in `get_engine()` method.
Added `DownloaderStats` pipeline middleware.
Refactoring. Import Headers, Request and Response classes from
crawlmi.http.
Added Engine into shell variables.
Created `Filter` pipeline middleware.