fix Issue #52

[wweggplant]

My jQuery version is 1.8. I find that if div.stuckMenu next element is empty,currentMarginT is NaN

 currentMarginT = parseInt($(this).closest('div').css('margin-top')) // line 64

it causes a problem that code does not work in line 100 ~ 103,

$('.stuckMenu').next().closest('div').css({
    'margin-top': stickyHeight + stickyMarginB + currentMarginT + 'px'
}, 10);