//Vertikaal centreren
function centerVertical(){
    if(navigator.userAgent.indexOf('Gecko') != -1)
        var h = window.innerHeight;
    else var h = document.documentElement.clientHeight;
    
    var content_h = document.getElementById('container').offsetHeight;
    var pos = (h/2)-(content_h/2);
    
    document.getElementById('container').style.top = pos + "px";
}