Duo authentication sample - initial commit

This is work in progress. This commit contains the Duo integration code
provided by Duo on GitHub.
Duo authentication work in progress...
Implementation of Duo security 2fa subflow
Duo subflow setup documentation
Typo
Text polishing
Added how to trigger
Duo authentication sample - initial commit

This is work in progress. This commit contains the Duo integration code
provided by Duo on GitHub.
Duo authentication work in progress...
Implementation of Duo security 2fa subflow
Duo subflow setup documentation
Typo
Text polishing
Added how to trigger
Merge branch 'issue#81' of github.com:Unicon/cas-mfa into issue#81
Merge branch 'master' into issue#81
Version chenage; Temp. disable checkstyle execution
Finalizing integration with ranking code from master
Refactor: Move Duo Java/Groovy code to its own Maven module
Bug fix: treat equal ranks as 'not stronger'