Added normal information to sampled points.

These changes forward the already computed normals to the outputted vtkPolyData.

The changes in vtkStratifiedSampling.cxx were straight forward. Main.cpp now uses an ArrowSource instead of the sphere and needed some extra processing (triangulation, normal splitting to avoid wrong interpolation over sharp edges). I also added the rendering of the Arrow mesh and normals to check that it's working. 

Have a look at it and let me know what you think.