Fixing a leak associated with collision delegate methods. (Thanks jjextra)

Former-commit-id: a2e0159

Author: slembcke

cocos2d/CCPhysicsNode.m

@@ -95,7 +95,7 @@ -(BOOL)ignore
 /// Internal class used to wrap cpCollisionHandlers
 @interface CCPhysicsCollisionHandler : NSObject {
 	cpCollisionHandler *_handler;
-	id _delegate;
+	__weak id _delegate;
 
 	// Cache the CCPhysicsNode's collision pair singleton.
 	CCPhysicsCollisionPair *_collisionPairSingleton;