[#544] new faster (and working) quadtree implementation based on Tino Hausmann library

this one can also be swapped with his spatial hash algorithm as they
both share the same API :)
[#544] QuadTree is now working and enabled by default !
[#544] couple of optimization to lower the creation of temporary variables
[#544] fixed the `maxChildren` and `maxDepth` parameters, and increased `maxChildren` to 8
[#544] added a collision handler and made the emoji bounce when colliding
[#544] keeping this one for later (probably 1.2.0)
updated CHANGELOG and README
make sure the body bounds are properly updated when manually adding objects

Note : this was working before, this is rather a safety net, if multiple
shapes are added manually :)
[#544] changed to a ellipse shapes as we are dealing with round emoticons ;P
Revert "[#544] changed to a ellipse shapes as we are dealing with round emoticons ;P"

This reverts commit 6b83b551fc00f903c15f210879c794ae2d85cfcd.
[#544] changed to an ellipse collision shape as we are dealing with emoticons