Update Article model to accept :tag_list attribute

 * We previously added fields for accepting 'tags' to our _form.html.erb
 file. When we try to submit the form, an undefined 'tag_list' method
 error is generated. Look in the xxx_create_articles.rb migration file
 and note that there are only attributes for :title and :body. So, it
 makes sense that an error would be shown when trying to create a new
 article with the additional :tag_list attribute.
 - To get around this, we define a 'tag_list' method within our Article
   model in the app/models/article.rb file.
 * Note, in order for the 'tag_list' method to return a comma separated
 list of tags, we had to re-define the 'to_s' method that .join uses. In
 this case to_s now returns the tag 'name'.