Remove not in use anylonger doorkeeper_for.
Remove unnecessary model_name hack in Application

Conflicts:
	lib/doorkeeper/models/application.rb
Redundant `self`s removed.
Rename authenticate -> by_token

Conflicts:
	lib/doorkeeper/models/application.rb
clean up models validates
More prominent notice in README about versions

[fixes #455]

[ci skip]
Allow revoked_at to be set in the future, for future expiry