Refactor Padrino Model Generator

Guys, as the title said I refactored the [Padrino Model Generator](https://github.com/padrino/padrino-framework/blob/6c8921c6c3f333d4beb3f7d24f89d3018b8ab558/padrino-gen/lib/padrino-gen/generators/model.rb).

I tried to make it simpler than before.
Please tell me if I made something wrong.

Hugs.
:v: on :earth_americas: