Update to schema definition syntax

[Łukasz Czyżykowski]

Currently sections are defined as local variables in the class scope
when defining schema. This approach works, but the sections are not
visually distinct from each other. Better approach would be to use
inner classes to achieve the scoping of options. An example:

    class ASchema(Schema):

        class database(Section):
            host = StringOption()
            port = IntOption()
            username = StringOption()
            password = StringOption()

        class login(Section):
            url = StringOption()
            access_token = StringOption()

instead of current:

    class ASchema(Schema):

        database = ConfigSection()
        database.host = StringOption()
        database.port = IntOption()
        database.username = StringOption()
        database.password = StringOption()

        login = ConfigSection()
        login.url = StringOption()
        login.access_token = StringOption()