The xmp tag doesn't have to be a child of body

See issue #27

The goal is to be able to do something like:

```html
<html>
<title>Title</title>
<body>
	<div id="main">
		<div id="sidebar"></div>
		<xmp style="display:none">
## Markdown
		</xmp>
	</div>
	<script src="strapdown/v/0.2/strapdown.js"></script>
</body>
<html>
```