Add importer for cached Google Reader RSS feeds

Google Reader is going bye-bye tomorrow, so I rushed to write something that could import posts from their cached RSS feeds (they look like "https://www.google.com/reader/atom/feed/$ENCODED_FEED_URL").

Not sure how much use this will be after tomorrow, but I hear the Archive Team is collecting these feed URLs for as many blogs as they can. So perhaps someone might have a need for this importer in the future? Figure it can't hurt. :)