Adding jaws.QuadTree

Adding quadtree functionality to JawsJS.

Using QuadTree.collide(), it will take as arguments two of any objects
that can be inserted into a SpriteList (or even two SpriteLists
themselves) and return if, at any point, they overlap with each other.

If the third argument, callback function, is set, collide will call that
function, with the two objects that collided as arguments, per collision
detected.