Add an octree.

[sebcrozet]

[bfops]

FWIW there's an octree available in playform. Parts of it might be usable.

[milibopp]

I'm working on an abstract library for spatial trees such as octrees and quadtrees. This may be useful for this issue, once it is a bit more fleshed out.

[viperscape]

It might be worthwhile adding a quadtree first, this would compose nicely with the 2d and 3d that ncollide provides. I may ask for a PR in the future, if I get up to that point in my development

[milibopp]

@viperscape, I would like to note, that acacia, the aforementioned library, is that generic by now. I'm not perfectly happy with the degree of abstraction yet, but it is definitely useful enough for this purpose.