refactor models' modules by Rails 4 way
separate ORM specifics from models
remove ORM specifics except active_record
Reverts test-related changes

We'll test ORM specific gem in doorkeeper's repository.
fix typos
let models handle including mixins
just rescue LoadError when loading ORM support
remove testing dependency of doorkeeper-orms
not loading ORM class in enable_orm function
Bring back ORM specific classes to doorkeeper
remove dependency of doorkeeper-orms and fix issues in ORM-specifics