getAngle = function(vector) {
            // get angle (clockwise) between vector and (1, 0)
            var x = vector[0];
            var y = vector[1];
            if (y >= 0) { // note that y axis points to its down
                return Math.acos(x);
            } else {
                return -Math.acos(x);
            }
        }
Adds the arcTo to the current path

@see http://www.w3.org/TR/2015/WD-2dcontext-20150514/#dom-context-2d-arcto