Code improvements noticed during PR review

[sergei-maertens]

This mostly comes from #16 which has turned into a massive PR, so we're splitting out the non-criticals here:

• Update the documentation - notably the settings have been removed (Project cleanup #20)
• Refactor fully-qualified URL building - there's mixed furl usage and string concatenation happening now which isn't particularly elegant
• Add to the documentation that the reverse proxy headers (:func:get_client_ip) need to be sanitized/trusted for client IP detection
• remove the requirements/base.txt and put all the requirements into setup.cfg metadata (pip install -e . needs to pull everything in) (Project cleanup #20)
• eherkenning requestedAuthnContext is now an ugly magic number list index - this should be fetched from config model
• convert tests to pytest style and replace mixins with pytest fixtures