Destroying microposts: Add delete link to the micropost on user's profile page, and delete link to the feed item on home page

Feed item is actually Micropost object (see User#feed), thus in 'feed_item' partial:
  <%= link_to "delete", feed_item, method: :delete, ... %>
acutal send DELETE request to /micropost/<id>
Destroying microposts: Add failing test for deleting microposts on home page
Destroying microposts: Get the test green by implementing MicropostsController#destroy and redirect to root_url after deleting

Techniques:
1. We defined a before filter to ensure that user could delete only his own microposts
2. In the 'correct_user' before filter, we find the microposts using code:
  @micropost = current_user.microposts.find_by(id: params[:id])
We use 'find_by' instead of 'find' because 'find' will raise exception if not found, thus if you'd like to use 'find', the code should be
  def correct_user
    @micropost = current_user.microposts.find(params[:id])
  rescue
    redirect_to root_url
  end
3. It might occur to you that we could implement the correct_user filter using the Micropost model directly, like this:
  @micropost = Micropost.find_by(id: params[:id])
  redirect_to root_url unless current_user?(@micropost.user)
This could be equivalent, but for security purpose it is a good practice always to run lookups through the association. Read http://www.rubyfocus.biz/blog/2011/06/15/access_control_101_in_rails_and_the_citibank-hack.html.