Called whenever the user is moving his/her fingers to scroll the view.
Updates the moveOffset so that the scroll-offset on the view is updated.
function _touchMove(event) {
        if (!this.options.enabled) {
            return;
        }

        // Process the touch event
        var primaryTouch;
        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) {

                    // When a thresshold is configured, check whether the move operation (x/y ratio)
                    // lies within the thresshold. A move of 10 pixels x and 10 pixels y is considered 45 deg,
                    // which corresponds to a thresshold of 0.5.
                    var moveDirection = Math.atan2(
                        Math.abs(changedTouch.clientY - touch.prev[1]),
                        Math.abs(changedTouch.clientX - touch.prev[0])) / (Math.PI / 2.0);
                    var directionDiff = Math.abs(this._direction - moveDirection);
                    if ((this.options.touchMoveDirectionThreshold === undefined) || (directionDiff <= this.options.touchMoveDirectionThreshold)){
                        touch.prev = touch.current;
                        touch.current = [changedTouch.clientX, changedTouch.clientY];
                        touch.prevTime = touch.time;
                        touch.direction = moveDirection;
                        touch.time = _getEventTimestamp(event);
                        primaryTouch = (j === 0) ? touch : undefined;
                    }
                }
            }
        }

        // Update move offset and emit event
        if (primaryTouch) {
            var delta = primaryTouch.current[this._direction] - primaryTouch.start[this._direction];
            this.updateScrollForce(this._scroll.touchDelta, delta);
            this._scroll.touchDelta = delta;
        }
    }