Add CommentsController

 * Run: rails g controller Comments
Create Comments form partial, _form.html.haml
Add Comment '_form' and include render partial call in Posts 'show' view
Add Comments as a sub-resource of Posts in config/routes.rb
Update PostsController 'show' method to instantiate new Comment object.
Add iterator to cycle through and display the comments for each post.
Add 'create' method to CommentsController.