Add a form and validation demo

[gkubisa]

http://semantic-ui.com/behaviors/form.html could possibly be used, but I think it's better to use one of the Elm modules:

• https://github.com/etaque/elm-simple-form
• https://github.com/rtfeldman/elm-validate
• https://github.com/shelakel/elm-validate
• http://package.elm-lang.org/packages/krisajenkins/elm-exts/12.7.0/Exts-Validation