Create CreditCard.java

        /* Pseudocode for isValid:
         * sum = 0
         * count = 0
         * for each digit starting from the right
         *     count ++
         *     if count is odd
         *         sum = sum + digit
         *     else if (digit < 5)
         *         sum = sum + 2 * digit
         *     else
         *         sum = sum + 2 * digit - 9
         * if the last digit of the sum is zero
         *     The card number is valid
         */