Use decorators instead of dict for db model "updaters"

[Tina-otoge]

let's have something like

class User(Model):
    @updater('email')
    def set_email(email):
        ...
        return email

instead of

class User(Model):
    UPDATERS = {'email': 'set_email'}

    def set_email(email):
        ...
        return email

[Tina-otoge]

Closed for inactivity