Form Helper

This is a WIP, please feel free to comment or suggest changes. An end goal for
me to is have something like
[SimpleForm](https://github.com/plataformatec/simple_form/) so hopefully
@josevalim will have some thoughts.

This will also rely heavily on the `Resource` protocol discussed in #320.
I wouldn't use a `do: block` here given that form_for does not use a `do: block` but a function. I would keep them consistent.