Form Helper

This is a WIP spike, 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 would just remove this altogether with the method check. If you want to treat any of those things as data, you need to move this logic up to the form_tag helper, it is not the job of the tag function.