fix: use layerX and layerY whith mouse events

Canvas container offset is ignored with clientX and clientY coordinates so nodes was searched in the wrong place on the canvas.
![2014-07-23 17 35 28](https://cloud.githubusercontent.com/assets/918009/3672331/e8eca872-125d-11e4-999d-1d4bc749b2c3.png)