function _quadTree(bounds, level, maxElements, maxLevel) {
    return {
      level: level || 0,
      bounds: bounds,
      corners: _geom.splitSquare(bounds),
      maxElements: maxElements || 20,
      maxLevel: maxLevel || 4,
      elements: [],
      nodes: []
    };
  }
Creates the quadtree object itself.

@param  {object}   bounds       The boundaries of the quad defined by an
                                origin (x, y), width and heigth.
@param  {integer}  level        The level of the quad in the tree.
@param  {integer}  maxElements  The max number of element in a leaf node.
@param  {integer}  maxLevel     The max recursion level of the tree.
@return {object}                The quadtree object.