decodes `document.forms` instead of `document`

Most significant is probably the change to work with document.forms instead #1

    <script>
        var app = Elm.Main.init({
            node: document.getElementById('myapp'),
            flags: {
-               document: document
+               documentForms: document.forms
            }
        })
    </script>

Next we've also bundled field and toNonEmptyString into NativeForm module itself. These were formerly helper functions written in the example app. Though other helpers were useful, especially toTimePosix, they were not included since it would increase the dependencies of this package. I suppose users can copy code from the demo app instead