Extract ORM specifics

rework of PR #454

ORM specifics except Active Record has extracted to [doorkeeper_bundle](https://github.com/jasl/doorkeeper_bundle) (not push to rubygems yet)
It was in models, for example <https://github.com/doorkeeper-gem/doorkeeper/blob/master/lib/doorkeeper/models/active_record/application.rb>

I think place here is better