Add ability to set a parent element for the player

I'm building a site and want a lightweight gallery -- this seemed to be a perfect easily extensible tool to use. The only thing I needed was the ability to specify where in the DOM I wanted to put the thing.

This option can be set just like the rest of the options, and requires a DOM element, e.g.
```javascript
shadowbox.options = {
  parentElement: document.getElementById( 'gallery' )
};
```