Added HTML5 dialog support

The `dialog` element for modals is a first class citizen in HTML5, with emerging browser support (http://blog.teamtreehouse.com/a-preview-of-the-new-dialog-element). 

I have changed the code such that `dialog` elements are styled like regular reveal modals, and when they have the `[open]` attribute (which browsers add when they have been opened through the native JS API) they are displayed.

However, I have also added it such that the dialog element can still be opened up using foundation's jQuery api (assuming they have `data-reveal` as an attr) . I have created mock files to demonstrate this.

If you have any problems I will be happy to fix. Thanks.