added ability to get the original url of a link from within a comment

Currently the reddit site has a link_url from a users overview page, whereas the JSON response does not. This makes it difficult to build a good experience around viewing a users profile. This pull request surfaces the link_url as a property. See http://www.reddit.com/r/redditdev/comments/1yj0x4/adding_link_url_property_for_comments/ for more information.