Consider removing marshmallow mixin from pony entities

[xakiy]

Consider removing marshmallow mixin from pony entities, and instead use pony own argument selector e.g. to_dict(exclude={'attr1', 'attr2']) or to_dict(only=['attr3', 'attr4']).

Currently we sanitize user input using marshmallow schema via mixin to pony entity. It need not be actually, because we can incorporate simple schema inside pony model. Thus making entity definition also contains json schema and validate user input by utilizing built-in before_insert method.