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.
`rescue NameError`
We should be sure to raise an `AdapterNotFound` error only if there is some problem with the class loading.

By catching generic exceptions, will swallow any error that `klass.new(mapper, uri)` could generate. In this case, a developer will see an `AdapterNotFound` exception and it will make hard to understand what's going on.