Add resource routing with skeleton controller and views for Articles

These changes configure conventional resource routing for the
ArticleController, with code skeletons for #index, #show, #new,
and #create. This commit also adds stubs for the corresponding views.
Build initial index view and controller method

Now the article index shows the 25 most recently updated articles.
For each article, we display the date the article was created, the
list of categories its been filed under, and the first 200 characters
of content.
Add show view and controller for individual articles

Now we can view the full content for individual articles. This view
is available through a hyperlink from each article title in the index.