More prominent notice in README about versions

[fixes #455]

[ci skip]
Allow revoked_at to be set in the future, for future expiry
Merge pull request #452 from radar/master

Allow revoked_at to be set in the future, for future expiry
refactor model's modules by ActiveSupport::Concern and move them into models/concerns folder
rename models' authenticate method
refactor scopes concern
wrap scopes concern(why not do would break mongoid 2 and 3???)
modulize doorkeeper model to decouple with orm
add fail tip when not found orm adapter