Quad Tree should only store objects on leaf nodes

[saliksyed]

Hey there -- thank you for the nice QuadTree implementation! However I'm noticing that there are often objects stored on the parent nodes as well as child nodes -- this shouldn't be the case.

See: http://stackoverflow.com/questions/4981866/quadtree-for-2d-collision-detection

[timohausmann]

I took another look at this issue and it has been resolved with the new version 1.2.0

#6