JSONValidatedField, model_service_factory, SimpleArrayField fix
Fixed basic support of JSONFields. Added JSONValidatedField that accepts ServiceObject against fields in which validation will be done. Added model_service_factory function to simplify creation of simple service objects for JSONValidatedField. Also fixed docs generation for SimpleArrayField used as parameter. Choices are being added as enum as well as 'initial' is preserved and used as 'default' for parameters