Make enter key on a form input trigger click on first submit button

When you press enter on an input inside of a form, the browser fires a submit event on the form, and also fires a click event on the first submit button of the form, if there is one. <button> elements default to type "submit" if a type is not specified.

This behavior is illustrated in this jsfiddle: http://jsfiddle.net/dkordik/y25r4qnp/

Closes #65