Added ArticleAdapter class.
Added RssArticle class.
Made class Article abstract in order to use polymorphism.
Minor tweaks and fixes for class Article and derived classes RssArticle
and RedditArticle.
Changed all instances of RedditArticle to Article.
Removed use of generics in ListListener.
Removed cast to base class Article in ListListener.
ListListener now uses base class reference for derived classes.
Added Article Feeds button which displays both RSS and Reddit articles
in a singular list view.
Added more informative comments on some files.
Miscellaneous tweaks, fixes, and updates.