This is a helper method that removes all observers with a particular
target. It can be used in destuctors to ensure no observers trigger
callbacks on objects that no longer exist.
1. update all copyright from 2010-2011 to 2010-2012 cocos2d-x.org
2. update COCOS2D_VERSION to 0x00020003
3. update const char* cocos2dVersion() to "cocos2d-2.0-x-2.0.3"