add mkt-prompt element

- Need to work on mobile/tablet

![screen shot 2014-12-18 at 6 05 00 pm](https://cloud.githubusercontent.com/assets/674727/5499226/7038bf22-86e0-11e4-9834-51fe33050cce.png)

@mstriemer @spasovski 
If you set this event to bubble then instead of calling `self.dismissModal()` in this handler you can register it on the element itself so that it is easier to add more buttons to trigger the dismissal. I think using events will make composing elements easier than calling methods in the event handler.

```html
<mkt-prompt>
  <a href="#close" class="close-prompt">Close</a>
</mkt-prompt>
```

```js
// In the element's prototype (the context of this comment).
self.addEventListener('mkt-prompt-cancel', function (e) {
    if (!e.defaultPrevented) {
        self.dismissModal();
    }
});
```

```js
// In the user's code.
$('.close-prompt').on('click', function (e) {
    $(this).trigger('mkt-prompt-cancel');
});
```