User Story 20, User can delete an author

[iandouglas]

As a Visitor,
When I visit an author's show page,
I see a link on the page to delete the author.
This link should return me to the book index page where I
no longer see this author listed.
If this author was the only author for any book, that book is also deleted.
If this author co-authored a book with someone else, that book should also be deleted, but not the other author.

(you may need to delete other content before you can delete an author or book)

DO THESE STORIES FIRST:

• User Story 14, Author Show Page

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 #9

child of #33

depends on #14