Simplify framework initialization

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`
- [ ] Make Lotus::Model to register adapters instances, and to specify a default one.
- [ ] Because of the previous requirement, consider to introduce a configuration and duplication mechanisms, like we do with Lotus::Controller and Lotus::View.
- [ ] Mapper should allow to specify adapters for a collection.
- [ ] Provide a single entry point to make the setup automatic.
The method signature should be unified with `Configuration#adapter`. Here is an optional param, there it's a named argument. I have no preferences about how to threat them, but we should make them consistent. :)