Called whenever the user releases his fingers and the touch gesture
has completed. This will set the new position and if the user used a 'flick'
gesture give the scroll-offset particle a velocity and momentum into a
certain direction.
function _touchEnd(event) {

        // Remove touch
        var primaryTouch = this._scroll.activeTouches.length ? this._scroll.activeTouches[0] : undefined;
        for (var i = 0; i < event.changedTouches.length; i++) {
            var changedTouch = event.changedTouches[i];
            for (var j = 0; j < this._scroll.activeTouches.length; j++) {
                var touch = this._scroll.activeTouches[j];
                if (touch.id === changedTouch.identifier) {

                    // Remove touch from active-touches
                    this._scroll.activeTouches.splice(j, 1);

                    // When a different touch now becomes the primary touch, update
                    // its start position to match the current move offset.
                    if ((j === 0) && this._scroll.activeTouches.length) {
                        var newPrimaryTouch = this._scroll.activeTouches[0];
                        newPrimaryTouch.start[0] = newPrimaryTouch.current[0] - (touch.current[0] - touch.start[0]);
                        newPrimaryTouch.start[1] = newPrimaryTouch.current[1] - (touch.current[1] - touch.start[1]);
                    }
                    break;
                }
            }
        }

        // Wait for all fingers to be released from the screen before resetting the move-spring
        if (!primaryTouch || this._scroll.activeTouches.length) {
            return;
        }

        // Determine velocity and add to particle
        var velocity = 0;
        var diffTime = primaryTouch.time - primaryTouch.prevTime;
        if ((diffTime > 0) && ((_getEventTimestamp(event) - primaryTouch.time) <= this.options.touchMoveNoVelocityDuration)) {
            var diffOffset = primaryTouch.current[this._direction] - primaryTouch.prev[this._direction];
            velocity = diffOffset / diffTime;
        }

        // Release scroll force
        var delta = this._scroll.touchDelta;
        var swipeDirection = (Math.abs(primaryTouch.current[0] - primaryTouch.prev[0]) > Math.abs(primaryTouch.current[1] - primaryTouch.prev[1])) ? 0 : 1;
        var allowSwipes = (swipeDirection === this._direction);
        this.releaseScrollForce(delta, velocity, allowSwipes);
        this._scroll.touchDelta = 0;
    }