Likely final commit

1. Assistance getting Book, Shelf, and Library Classes to relate to each
other. (Library knows about shelves. Shelf knows about books.)  (Vijay)
2. Assistance with shelf and unshelf methods, and num_shelves attribute
(Vijay)
3. Consistent naming convention (Margie)
4. Add add_shelf() function (Margie)
5. Comments (Margie)
6. Format list_books() function (Margie)
7. To do list (i.e. learning goals) (Margie)