Add XSL

Resolves issue 1411.

English only. Though we could specify per-language stylesheets later when we find a good way to manage the translations. Progressive enhancement.

cc/ @Kwpolska @ralsina 
What is the problem with just doing `rss_obj.xsl_stylesheet_href = self.url_replacer(feed_url, "/assets/xml/rss.xsl")` above?  Also, this function seemingly overwrites itself with a string.