Allow normals on shadowed surfaces

Changed ambient for use a factor of light with camera direction, this allow normals on shadowed surfaces.

Before: 
![screenshot 2014-12-08 20 54 47](https://cloud.githubusercontent.com/assets/8334203/5369077/a494c988-7fdc-11e4-92bc-a1c74fe023ea.jpg)

After:
![screenshot 2014-12-08 20 54 58](https://cloud.githubusercontent.com/assets/8334203/5369079/ab464a04-7fdc-11e4-9ab5-eafaca9011ba.jpg)

(on behalf of Luis)