Frustum culling

Added the ability to do frustum culling
It's maths :). 
A point outside the frustum should accomplish dist = ax+by+cz+d <=0
for a bounding sphere is ax+by+cz+d+r<=0 
So changing r to the right side of the equation dist <= -r

You can check http://www.crownandcutlass.com/features/technicaldetails/frustum.html or 
http://zach.in.tu-clausthal.de/teaching/cg1_0607/literatur/lighthouse3d_view_frustum_culling/index.html 

For example