Example: Better Support for 100% Height Overlay

The overlay on the example page at http://usablica.github.com/intro.js/ doesn't provide support for 100% height of the browser, just 100% height of the content.  

1. Content < Browser Height: By specifying that both `html` and `body` should utilize 100% of the browser's window, the overlay will also inherit the full height of the browser window.

2. Content > Browser Height: After setting the `html`, `body` height to 100%, there is a second problem to solve.  If the content is greater than the browser height, scrolling the window causes the overlay to cut off at "the fold".  By setting the overlay CSS position property as "fixed", it will ensure that scrolling the browser window will maintain the overlay covering the content of the page.

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This pull request prevents:

![Screen Shot 2013-03-15 at 3 57 18 PM](https://f.cloud.github.com/assets/126483/265920/b8275a68-8dc3-11e2-8c94-35f0c571cf04.png)