Add the 'update' action to ArticlesController

 * Just like the 'new' action sends its form data to the 'create'
 action, the 'edit' action sends its form data to the 'update' action.
Add 'flash update' to the update action in ArticlesController

 * Users will be shown a message letting them know that the article was
 changed.
 * Note, we still need to add the 'flash.notice' object to our views,
 otherwise the message will not be shown.
Add a 'flash notice' to our application layout

 * The application layout wraps our view templates (such as 'edit',
 'new' and 'show').
Add a flash notice to controller 'create' and 'destroy' actions
Set root to 'articles#index'