handle future pubDate

Posts with pubDate in the future always appear at the top, which is ruining the
whole "newest at top" idea. One way to handle this is to use time() instead.
This is what google reader does and seems reasonable.

One example of such a feed is http://www.rss-specifications.com/blog-feed.xml. It currently (Mon, 25 Mar 2013 16:56:10 +0000) has a post with pubDate in the future:
```
<pubDate>Thu, 28 Mar 2013 09:00:00 -0400</pubDate>
```