function $(s) {
    return( document.getElementById(s) );
}

function encode_dict(d){
    var r = [];
    for(var k in d){
        r.push(encodeURIComponent(k) + '=' + encodeURIComponent(d[k]));
    }
    return r.join('&');
}

function query(url, params, callback){
    var xhr = new XMLHttpRequest();
    xhr.open('GET', url + '?' + encode_dict(params));
    xhr.onreadystatechange = function() {
        if( xhr.readyState === 4){
            callback(xhr.responseText);
        }
    };
    xhr.send();
}