changed test 'it "has a default status of available" to pass in title, author as arguments to book init
revert test code to init book with "Book = book.new"; supplied default arguments in Book::initialize instead
implement Book::check_out method that updates @status instance var to the string "checked_out" and returns true
Added conditional validation to Book::check_out method.  Only update book.status to 'checked_out' if book is still 'available'.
implement Book::check_in method
Borrower::initialize now correctly sets instance var 'name'.  Book::name method returns name instnce var
Library::register_new_book creates new book object, adds that to @books array, incrememnts book counter (for unique id), and sets the book object id to the counter value
implement Library::check_out_book - allows Borrower to check out book via id
add @borrower attribute to Book class, Library::check_out_book sets borrower object on book instance, and Library::get_borrower returns borrower's name
remove Borrower.backpack
remove _a? typo fom library_spec.rb
re-factor Library::register_new_book method to take book object, rather than title & author and instantiating a book object itself.
Library::check_out_book returns nil if book's status is 'unavailable'
borrowers can now check in books using Library::check_in_book
changed test to check for borrower max of 2 books at a given time
Library::check_out_book method now checks if a borrower's book count allows for the check out (less than 2)
Library::available_books returns count of array of books with 'available' status
changed test on library_spec.rb:216 to 'lib.available_books' instead of 'lib.borrowed_books'... kors was trying to borrow from an empty set