started implementing backfreferences from voxel to voxeltreenode
merge with master
merge with master
fixed tests
merge with master
add crucial voxel to planet
Merge branch 'master' of https://github.com/voxelinc/voxellancer into voxeltreenode-ref-in-voxel

Conflicts:
	src/voxel/specialvoxels/cockpitvoxel.h
	src/voxel/specialvoxels/enginevoxel.h
	src/voxel/specialvoxels/fuelvoxel.h
Merge pull request #174 from voxelinc/voxeltreenode-ref-in-voxel

Add VoxelTreeNode Pointer to Voxel, speeds up removal from CollisionDetector