Add attributes class method to Ohm::Model

This adds an `attributes` class variable and method to Ohm::Model that consists of an array of `attribute` keys. This is useful for scaffolding or if you need to know all your attribute keys and have an empty or partial instance. 

More specifically, I need this for Padrino's admin app generator. [Take a look at this issue for more info](https://github.com/padrino/padrino-framework/issues/1148).

Thanks,
-Ty.