Break strong reference cycle in RMQuadTreeNode.

@@ -48,7 +48,7 @@ typedef enum : short {
 @property (nonatomic, readonly) RMProjectedRect southWestBoundingBox;
 @property (nonatomic, readonly) RMProjectedRect southEastBoundingBox;
 
-@property (nonatomic, readonly) RMQuadTreeNode *parentNode;
+@property (nonatomic, weak, readonly) RMQuadTreeNode *parentNode;
 @property (nonatomic, readonly) RMQuadTreeNode *northWest;
 @property (nonatomic, readonly) RMQuadTreeNode *northEast;
 @property (nonatomic, readonly) RMQuadTreeNode *southWest;

@@ -68,7 +68,8 @@
 {
     RMProjectedRect _boundingBox, _northWestBoundingBox, _northEastBoundingBox, _southWestBoundingBox, _southEastBoundingBox;
     NSMutableArray *_annotations;
-    RMQuadTreeNode *_parentNode, *_northWest, *_northEast, *_southWest, *_southEast;
+    __weak RMQuadTreeNode *_parentNode;
+    RMQuadTreeNode *_northWest, *_northEast, *_southWest, *_southEast;
     RMQuadTreeNodeType _nodeType;
     __weak RMMapView *_mapView;