remove unused helper for submitButton
Removed display of unsaved record with null id in list of posts and sort by newest posts first.
See discussion at http://stackoverflow.com/questions/14705124/creating-a-record-with-ember-js-ember-data-rails-and-handling-list-of-record
Note change from iterating over "each model" to iterating over "each post in filteredContent" in index.html.erb. That change
requires attributes be referenced by "post", and the updated linkTo takes the route, "post", as well as the "dynamic segment" which is also named "post",
per the above #each post.  (refer to http://emberjs.com/guides/templates/links/).
Note the addition of the PostsController. Previously, it was implicitly defined. It listens to property "arrangedContent.@each" so that when
the new post saves, the filteredContent property updates and notifies the view template using this property in index.html.erb.
Without the listener on this property, the view of all posts would not update.