Implement QuadTree for Collision Detection.

[cantsin]

• start quadtree.lua: new data structure. implement a node class containing aabb, a list of nodes, and a list of objects
• implement a function to split given node in four equal parts (returns four nodes)
• implement clear function: resets all list of objects in all nodes to empty list
• implement insert function: figure out which node can fit the given sprite and insert the sprite into the node.
• implement retrieve function: given an object, return possible collisions.

Here's a possible reference implementation (in javascript): http://gamedevelopment.tutsplus.com/tutorials/quick-tip-use-quadtrees-to-detect-likely-collisions-in-2d-space--gamedev-374