Called whenever the user starts moving the scroll-view, using
touch gestures.
function _touchStart(event) {

        // Create touch-end event listener
        if (!this._touchEndEventListener) {
            this._touchEndEventListener = function(event2) {
                event2.target.removeEventListener('touchend', this._touchEndEventListener);
                _touchEnd.call(this, event2);
            }.bind(this);
        }

        // Remove any touches that are no longer active
        var oldTouchesCount = this._scroll.activeTouches.length;
        var i = 0;
        var j;
        var touchFound;
        while (i < this._scroll.activeTouches.length) {
            var activeTouch = this._scroll.activeTouches[i];
            touchFound = false;
            for (j = 0; j < event.touches.length; j++) {
                var touch = event.touches[j];
                if (touch.identifier === activeTouch.id) {
                    touchFound = true;
                    break;
                }
            }
            if (!touchFound) {
                //_log.cal(this, 'removing touch with id: ', activeTouch.id);
                this._scroll.activeTouches.splice(i, 1);
            }
            else {
                i++;
            }
        }

        // Process touch
        for (i = 0; i < event.touches.length; i++) {
            var changedTouch = event.touches[i];
            touchFound = false;
            for (j = 0; j < this._scroll.activeTouches.length; j++) {
                if (this._scroll.activeTouches[j].id === changedTouch.identifier) {
                    touchFound = true;
                    break;
                }
            }
            if (!touchFound) {
                var current = [changedTouch.clientX, changedTouch.clientY];
                var time = _getEventTimestamp(event);
                this._scroll.activeTouches.push({
                    id: changedTouch.identifier,
                    start: current,
                    current: current,
                    prev: current,
                    time: time,
                    prevTime: time
                });

                // The following listener is automatically removed after touchend is received
                // and ensures that the scrollview always received touchend.
                changedTouch.target.addEventListener('touchend', this._touchEndEventListener);
            }
        }

        // The first time a touch new touch gesture has arrived, emit event
        if (!oldTouchesCount && this._scroll.activeTouches.length) {
            this.applyScrollForce(0);
            this._scroll.touchDelta = 0;
        }
    }