function for making Ajax requests.
function makeRequest(url, callback) {
var request;
if (window.XMLHttpRequest) {
request = new XMLHttpRequest(); // IE7+, Firefox, Chrome, Opera, Safari
} else {
request = new ActiveXObject("Microsoft.XMLHTTP"); // IE6, IE5
}
request.onreadystatechange = function() {
if (request.readyState == 4 && request.status == 200) {
callback(request);
}
}
request.open("GET", url, true);
request.send();
}