follow rails autoloading rules

We began getting a `ActiveSupport::Concern::MultipleIncludedBlocks`

- `Model::Declarable` -> `Declarable`
- `Models::Formatter` -> `Formatter`
Merge pull request #215 from bitzesty/minor-rails-enhances

follow rails autoloading rules
More rails autoloading fixes

formatter and declarable are now autoloaded, remove `require` to avoid

`ActiveSupport::Concern::MultipleIncludedBlocks` errors