angle

distance: function(x1, y1, x2, y2) {
    return Math.sqrt(Math.pow(x1 - x2, 2) + Math.pow(y1 - y2, 2));
},
angle: function(x1, y1, x2, y2) {
    var x = x1 - x2;
    var y = y1 - y2;
    var l = Util.distance(x1, y1, x2, y2);

    // var result = Math.atan(x / y) / (-Math.PI * 2);
    // var result = (Math.acos(y / l) / (Math.PI * 2)) * 360;
    var result = Math.acos(y / l);

    return result;
}