Put RSS articles into sandboxed iframes, to prevent XSS problems

XSS type problems I've seen:

 1. CSS leaking from the article onto the reader app
 2. Javascript messing up the reader app

An example of 2. is <http://feeds.feedburner.com/smbc-comics/PvLb>, where each article contains the following script tag:

````html
<script type="text/javascript">
var gaJsHost = (("https:" == document.location.protocol) ? "https://ssl." : "http://www.");
document.write(unescape("%3Cscript src='" + gaJsHost + "google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E"));
</script>
<script type="text/javascript">
try {
var pageTracker = _gat._getTracker("UA-3727700-1");
pageTracker._trackPageview();
} catch(err) {}</script>
````

Calling `document.write` like this causes the entire page to go blank.

The `iframe` contains the `sandbox` attribute and disables javascript entirely inside the `iframe`.

----

The `iframe` tags as generated in this pull request automatically gets sized to fit the contents in the vertical direction.

When navigating inside the `iframe`, that navigation takes effect inside that `iframe`, and no additional vertical resize is effected. This is a little weird, and I'm not entirely sure what to do about it. I recommend navigating to the article with the button in the reader interface.