make a failing test for reviewable inactive books

tests in features/books
chang book factory to be a sequence
add book.reviewable?
make inactive reviewable test pass

this is only checking 'reviewable', not behaviour of a reviewable book
add require_reviewable_book to review

also clarified spec
Merge branch '14-no-review-deactivated' into 14
check for book.is_active before new review

also pull a useless private function out of review model
add failing test for review_inactive to book_spec

OK, so simplecov is showing 2 untested lines:
app/controllers/reviews_controller.rb:18-19
which redirect if the book is inactive.
Interactive testing in the browser shows this works as expected.

It is my test that is incorrect.
spec/features/books_spec.rb:59
tries to visit the page and click "New", but capybara is not seeing the
"New" button.
Can someone pull this locally and figure out the test?
pass tests for #14

test reviews_controller_spec
put in an HTTP_REFERER note
commented out the test that I liked, but couldn't fix, in features books
Merge pull request #93 from nbirnel/14

14