Made the notification center instance be created only when an observer is first added (aka lazily)
Made the notification center instance be created only when an observer is first added (aka lazily)

cureHsu/Objective-LevelDB,matehat/Objective-LevelDB,uber/Objective-LevelDB,cybertk/Objective-LevelDB,lsm/LevelDB-ObjC,uber/Objective-LevelDB,cybertk/Objective-LevelDB,cureHsu/Objective-LevelDB,fullstackclub/Objective-LevelDB,cureHsu/Objective-LevelDB,jinjian1991/Objective-LevelDB,uber/Objective-LevelDB,yincheng1988/Objective-LevelDB,i-miss-you/Objective-LevelDB,cureHsu/Objective-LevelDB,jinjian1991/Objective-LevelDB,lsm/LevelDB-ObjC,lsm/LevelDB-ObjC,fullstackclub/Objective-LevelDB,i-miss-you/Objective-LevelDB,appunite/Objective-LevelDB,cybertk/Objective-LevelDB,jinjian1991/Objective-LevelDB,i-miss-you/Objective-LevelDB,uber/Objective-LevelDB,akkakks/Objective-LevelDB,yincheng1988/Objective-LevelDB,i-miss-you/Objective-LevelDB,appunite/Objective-LevelDB,fullstackclub/Objective-LevelDB,matehat/Objective-LevelDB,appunite/Objective-LevelDB,matehat/Objective-LevelDB,yincheng1988/Objective-LevelDB