Nested namespaces

This PR adds the ability to us create a settings class with a nested namespace, so the "base hash" can be fetched from inside some yaml key.

```
==== Namespaced settings

Maybe you need to use another semantics for your namespace besides the current
environment. You can put a nested namespace in your settings and you are good
to go.

  # /some/file/here.yml
  professional:
    preferences:
      order: desc
  client:
    prefs:
      order: asc

  class ProfessionalPreferences < Settingslogic
    source "/some/file/here.yml"
    namespace "professional.preferences"
  end

  >> ProfessionalPreferences.order
  => "desc"
```