Enable both Date#to_era and Time#to_era by just requiring 'era_ja'

To do so, move main logic to `EraJa::Conversion` module.

Currently when we work on a project with bundler, we have to load `era_ja/date` and `era_ja/time` manually. It would be useful if we can use `to_era` method by just writing `gem 'era_ja'`.