Save cover color so book does not change each run

- BookInfo loses the (random, generated) cover color property
- Code for generating random cover colors moves to Book
- BookServer is responsible to create one for a book that doesn't
  have it already
- Places that used to set it no longer need to
Refactor and cleanup after code review
Merge pull request #149 from JohnThomson/saveCoverColor

Save cover color so book does not change each run