/*
Find the angle of the two vectors, solving the formula for
the cross product a x b = |a||b|sin(θ) for θ.
@param {Number} x the x-coordinate
@param {Number} y the y-coordinate
@return {Number} the angle in radians
function(x, y) {
        return Math.atan2(
            this.x * y - this.y * x,
            this.x * x + this.y * y);
    }