Add articles

Allow visitors to create and view all existing articles. Resolves #2. Very basic functionality and views at the moment.
Personal pet peeve -- indenting methods under `private`.  Since it's just a [class method mixed in from `Module`](http://ruby-doc.org/core-2.1.2/Module.html#method-i-private), I follow the same indentation rules as when I use something like `has_many`.  Your call.