Refactoring article.rb. Extracted methods to new classes.

Created Goldencobra::ArticleType and Goldencobra::ListOfArticles to
comply with the Single Responsibility Principle. Goldencobra::Article
isn't responsible for List_of_articles and article_type anymore.
prepare to refactor artocle & related object
refactored related_object calls to a new RelatedObject model

several refactors to avoid if/else
Refactor link checking into new class. Keeping article clean.

Article doesn't need to know how to check links. Article should only
know about itself. A new class named Goldencobra::LinkChecker can be
used for that instead. This way Goldencobra::Article stays clean and
obeys the Single Responsibility Principle.