engine.py: Fixes typo in docstring.
Moves http.RedirectTracker, http.RichClient to web module.

Removes RobotsTxtRichClientSessionMixin.
Adds docs for web module.
Refactors Engine and Processor interaction to be asynchronous.

Moves request client concern out of Engine and into Engine.
Engine handles stats start and stop.
Moves cookie, robots, redirect concerns out of WebProcessor.
conversation.py: Adds some docstring.
changelog: Adds entry about API Engine and Processor refactor.