first commit
alter check_out method for book class
add check_in method
make test for check_in work
run test for borrower's name
Runs first test on library
define register_new_book method and run test
activate test for multiple books
add checkout by book id method and run test
add borrower key to book object, add method to get borrower by book id, run assoc test
append check_out_book to return nil if book is already checked out
add method to let a borrower check it back in
add borrowed array to borrower class and append checkout method to verify less than two books borrowed
add available books array and method to search for it
add borrowed books array and alter test to check for available books