DSL to register and load adapter instances

This PR aims at a complete implementation of the feature set requested by https://github.com/lotus/model/issues/33

- [x] Instantiate global mapper through `Lotus::Model.configure`
- [x] Make Lotus::Model to register adapters instances, and to specify a default one.
Won't this break with the default value of `{}` as `{}.default` won't respond properly?