Replaced invalid_resource_owner error with invalid_grant

Issue #444 - error code "invalid_resource_owner" is not in the standard
Update CHANGELOG. [ci skip]

Fixes #450.
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