add sort rating, close (#25), reopen (#4)(#5)(#6)

So this breaks the ratings we had.
But... the logic is there for ratings.
It is the UI which is missing.
So I would like to merge this in.
It does break 4 reviewscontroller tests, but we shall see how that works
with master.

app/controllers/books_controller.rb
    default sort in rating
app/controllers/reviews_controller.rb
    add sort_column helper method
app/helpers/books_helper.rb
    def avg_rating_view
app/helpers/reviews_helper.rb
    def rating_view review
app/models/book.rb
   def update_rating!
app/views/books/index.html.haml
      %th= sortable "rating"
        %td= rating_view book
app/views/books/show.html.haml
   = avg_rating_view @book
app/views/reviews/_form.html.haml
      ~#= render 'rating_stars.html.haml', review: @review, book: @book
      = number_field_tag 'quantity', nil, in: 1...5
      FIXME this doesn't work!
app/views/reviews/index.html.haml
    use lc field headers
app/views/reviews/show.html.haml
  = rating_view @review
spec/features/books_spec.rb
  describe 'Showing' do
    describe 'an unrated book' do
      it 'shows no ratings' do