Improvements for the database layer.

Including a test case, moving application settings to an external file,
enabling find delete operations directly from Model classes, support for
"blank" and "default" field attributes, inclusion of a StringField,
support for field-specific storage codification, support for custom
Field validation with proper custom exception, detection of model class
redefinition.

The database layer still needs further testing, and custom fields need
to be improved for validating lenght ranges.