Fix crashing bug in voxel-server related to animation
- added VoxelNodeDeleteHook mechanism that allows users of VoxelNode to register for delete
  notification call backs
- added call back to VoxelNodeBag to remove and nodes from bags when the nodes get deleted.
removed bogus code that related to server crash
Merge pull request #699 from ZappoMan/voxel_animation

Fix crashing bug in voxel-server related to animation