handle multiple images within the article body

Hey there, I wrote this to deal with [an article](http://calendar.perfplanet.com/2013/web-performance-waterfalls-from-10000-feet/) that really needed the images. Then I noticed your #13 and thought it might be useful. 
It limits the number of articles by the age (in days) of the oldest article. The default is 7.0, so this line doesn't alter your functionality; it only makes the setting explicit for anyone who wants to adjust it. Reference is [here](http://manual.calibre-ebook.com/news_recipe.html#calibre.web.feeds.news.BasicNewsRecipe.oldest_article).