Cleanup of CCSpriteSheetCache
Cleanup of CCSpriteSheetCache

* renaming of singleton property to Instance for consistency
* replace method SpriteSheetForKey with Indexer

hig-ag/CocosSharp,netonjm/CocosSharp,MSylvia/CocosSharp,zmaruo/CocosSharp,mono/CocosSharp,TukekeSoft/CocosSharp,netonjm/CocosSharp,mono/CocosSharp,TukekeSoft/CocosSharp,haithemaraissia/CocosSharp,zmaruo/CocosSharp,MSylvia/CocosSharp,haithemaraissia/CocosSharp,hig-ag/CocosSharp