Refactor internal setting validation

[ziima]

Trying to implement a MappingSetting (#94), we encountered a problems with validation of a custom nested settings. Current implementation relies on exceptions for NestedListSetting and NestedDictSetting in Setting.raw_value.

We need to refactor the validation or the value extraction to support nested settings in general. One of the main problems is that we can't pass a setting value or its part to a nested setting instance.