Get container position on mouse down

Nobody has claimed this yet.

There is problem when get contianer position at componentDidUpdate:

When container scroll out of visual range then update position, scroll back, we can't call handleMouseMove correctly

Why not let it work onMouseDown.

`handleMouseDown: function handleMouseDown(e, index) {
this._draggingIndex = index;
this._prevX = e.pageX || e.clientX;
this._prevY = e.pageY || e.clientY;
this._initOffset = e.offset;
this._isReadyForDragging = true;
this._hasInitDragging = false;

var container = _reactDom2['default'].findDOMNode(this);
var rect = container.getBoundingClientRect();

var scrollTop = doc.docElement && doc.docElement.scrollTop || doc.body.scrollTop;
var scrollLeft = doc.docElement && doc.docElement.scrollLeft || doc.body.scrollLeft;

this._top = rect.top + scrollTop;
this._left = rect.left + scrollLeft;
this._bottom = this._top + rect.height;
this._right = this._left + rect.width;

// start listening mousemove and mouseup
this.bindEvent();
}`

Start at the handleMouseDown entry point and compare it with the componentDidUpdate behavior described in the issue. Check that the container bounds are captured on mouse down and that dragging still handles mouse movement after the container has been scrolled out of view and back.

Written by the indexing model from the issue text.