User Story 8, Book Index Page can be Sorted

[iandouglas]

As a Visitor,
When I visit the book index page,
I should see one link each to sort the books by the following criteria:
- sorted by average rating in ascending order
- sorted by average rating in descending order
- sorted by number of pages in ascending order
- sorted by number of pages in descending order
- sorted by number of reviews in ascending order
- sorted by number of reviews in descending order

Checklist:

• base code tests are written
• base code is written to pass tests
• edge case ("sad path") tests written
• edge case code written
• code is reviewed
• branch is merged
• all tests still pass

child of #26

child of #33

depends on #25