Added Articles routing spec whilst tracking down another problem.
Added #show method to Article controller (and spec), along with view (and view spec) for it.

This uncovered some bugs and omissions:
- Added LinkItem#to_html method and spec; formats item's URL and text into a standard HTML anchor tag pair.
- Fixed howling bug in spec for ArticleDecorator#format_links; method's code now uses LinkItem to build anchor tags for return value.