Support Shadow scaling for CanvasZoom and ScaleFactor

This should account both for scale factor and canvas zooming

current fabricjs behaving
![image](https://cloud.githubusercontent.com/assets/1194048/4742343/378745fc-5a1e-11e4-8b12-19452de1ee32.png)

with this pr everything zoomed
![image](https://cloud.githubusercontent.com/assets/1194048/4742296/dd1f689c-5a1d-11e4-9fac-1a5f1616b355.png)