Refactor models

This a big refactor

- use rails4-way refactoring model modules
- rename `authenticate` and make more clearly
- remove `Application` model's `model_name` magic
- extract model's non-ORM-specify codes to module(a way to decouple ORM and model)
- some clean up

The PR may not be merged, but may give some ideas to disscussion 
Place the . on the previous line, together with the method call receiver.