Add resource routing with skeleton controller and views for Categories

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

Now the category index shows a list of all categories in alphabetical
order. For each category, we display the number of articles it contains.
Add show view and controller for individual categories

Now we can view category detail, including a list of all articles
filed under a given category. This view is available through a
hyperlink from each category name in the index.
Add links to category views from the article views

The category views are now accessible via hyperlinks from the article
index and show article views.
Alter the welcome page to show the five most recent articles

The default, welcome page displays the five most recently updated
articles in descending order, with links to the individual articles
and to their respective categories.