Created a sample Form Login class.

Simple class to show how a custom authentication provider can use Duo
Security two factor authentication.
Made 'duo' section optional in config

Avoids prompting the user for config data when the bundle is enabled
but is not in actual use.
DuoWeb now accepts UserInterface objects
Moved controllers to own config file
Added login/logout capabilities

Created a login screen for the sample form login provider to use when a
custom authentication provider is not necessary.
Configured default check_path for Duo
Changed auth position to pre_auth
Created duo_security option for login config

A dedicated option is now used to specify whether or not two factor
authentication should be used.
Fixed remember_me capabilities