Added date to the articles

Hi Steve,

I've added a very basic date to the articles.  This uses the 'date' metadata, which not all of the articles have (in which case it will just be blank).

The format I've chosen for dates is "%-d %B %Y" - you can change this if you want in the formatted_date helper in blog.rb.

Also, it's 'barely' styled.  You can set the style for the date in layout.css

-Chris