this fixes the wrong positioning

solution for #50

I wanted to show some intros in a div, but intro.js positioned the inserted elements wrong. Intro.js for me calculated the positions for the elements relative to the body.

Have to mention if i call like this <code>introJs('.some_element').start();</code> and these elements contains the data attributes, than nothing happen (readme says to use like this).
So i called the Intro.js on the parent of these elements, which worked great, but the positioning is wrong (and the parent isnt the body).

This is my solution for the problem.

Its require more testing, for me working well just had no time to test in more situations.

Maybe you should add this as well, so the overlay hide the whole html (without this only cover the parent element)

```css
position:fixed;
left:0;
top:0;
width:100%;
height:100%
```