update
	*smoother error msg transitions
		~3 types of errors: luhn, digit, expiration
		~back button always point to previous selection
		~disable buttons on error (gives only option to press back btn)
		~credit card # and exp. date retain all data in any case except:
			-when the card processes (either approved or rejected)
	*added disabled and enabled buttons methods
		~enableButtons(): enables buttons
		~disableButtons(): disables buttons