Added validation before checkout button click

[rsinghcodes]

Fixes #24

I have added card validation before clicking on checkout button. Please review it and merge it.

Note: Below code is unused and will not affect the appearing of the modal box either. So, I removed it.

window.onload = function() {
    setTimeout(function() {
        modal.style.display = 'block';
    }, 3000);
}

function check() {
    modal.style.display = "block";
}

Thanks!