Adds methods to display surfels/quad with normal vectors

- Ok for both Viewer3D and Board3D.
- usage  should be similar to 

         Display3DFactory<>::drawSurfelWithNormal(viewer,
                                               surfel,
                                               normalVector);
   
- surfel being a Khalimsky cell (signed or not) and normalVector a unitary normal vector.
 

Illustration with a 100^3 digital sphere surface with "exact" normal vectors:

![snapshot](https://cloud.githubusercontent.com/assets/700165/2699552/eaf735d2-c407-11e3-91b5-b3b111ce92b3.jpg)


TODO:
 - assert that the kcell/cell is a 2-cell 

BE CAREFUL : with QGLVIEWER, you need to press "D" to enable double-face rendering