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.
`raise AdapterNotFound.new(class_name)`

```ruby
class AdapterNotFound < ::StandardError
  def initialize(adapter_name)
    super "Cannot find Lotus::Model adapter #{ adapter_name }"
  end
end
```

When an application has multiple adapters, for debugging purposes, it's useful to understand which is the missing adapter.