Use <button> instead of <input type="button">

Fixes this bug: if you look at http://elm-lang.org/edit/examples/Intermediate/Form.elm in WebKit, at least, the button is always the same height right now, no matter what height / size you put on in Elm.

A little research shows that sticking with <input> is fraught with problems and requires CSS hacking if you want to customize height:
http://stackoverflow.com/questions/1682520/css-how-to-increase-the-size-of-a-osx-submit-button
http://stackoverflow.com/questions/10976700/how-to-set-the-height-of-an-button-input-element-on-webkit-browsers
http://stackoverflow.com/questions/6184210/how-can-i-control-the-height-of-text-inputs-and-submit-buttons-in-different-brow

<button> is allowed to have customizable height and is otherwise identical to <input> in all modern browsers. (You could also embed HTML / styled text inside the button later, if you wanted.)