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
Bring back ORM specific classes to doorkeeper
not loading ORM class in enable_orm function
remove dependency of doorkeeper-orms
Minor adjustments
Transform comments into tests in rescue block
Merge pull request #491 from doorkeeper-gem/pr/478

ORM refactors