setZOrder: in CCNode.m

setting the zOrder of the current node is redundant if parent is present and will directly interfere with if( z == child.zOrder ) check in reorderChild:z: from CCSprite, CCSpriteBatchNode and CCParticleBatchNode causing them to defer from reordering their children. 

this is related to issue 598.
Merge pull request #604 from kalo-/patch-1

updated setZOrder: in CCNode.m