A better solution for the javascript part, since the one given only supports a single spinner per page.
$('.spinner .btn:first-of-type').on('click', function() {
var spinner = $(this).parent().parent().find('input');
spinner.val(parseInt(spinner.val(), 10) + 1);
});
$('.spinner .btn:last-of-type').on('click', function() {
var spinner = $(this).parent().parent().find('input');
spinner.val(parseInt(spinner.val(), 10) - 1);
});
A possible update is to add min, max, scroll and keyboard support.