Initial work towards rendering voxels with different colors.
Merge remote-tracking branch 'origin/master' into colored_voxels
Functional support for individual coloring of voxels.

Voxel colors get written into the lowest level of the octree (not
textured).
VoxelPipe shader colors all voxels green (for now).
Only supports voxel coloring with OpenGL (not CUDA yet).
Merge pull request #6 from wang1262/colored_voxels

Colored voxels