Swap property method and :as option

This is not ready for pulling, but I'm opening a PR now so I can get some feedback on this.

I have
* updated the tests
* renamed the option from :private_name to :as
* updated `create_accessor` to make a delegate method with the aliased name if it exists. Not sure if this is right.

Should Fields use the aliased method name or the real method name? I kind of feel that Fields should have the real name, and that Form should have a method that delegates the aliased name to the real name on Fields. Or maybe, since Fields seems to be using Representable to do the to_hash stuff, the :as option should be passed in to the Representor so it can do it's usual :as magic.

How do you want it done?