Markdown.Sanitizer.js should not use global window variable

```
We are trying to use Markdown.Sanitizer.js in Acre (a server-side javascript
environement).

However, we find ourselves hacking a window object to use Markdown.Sanitizer.js.

But if Markdown.Sanitizer.js passes "this" as the "window" variable, we can use
Markdown.Sanitizer.js on the server-side as is:

(function(window) {

...

})(this);

```

Original issue reported on code.google.com by `daep...@google.com` on 24 Sep 2012 at 2:59