__Dictionary is the implementation of CCDictionary. Because we changed CCDictionary to Dictionary in alpha or beta version. Then we want to deprecated Dictionary. But because developers will update from v2.x to 3.0, so we should deprecated CCDictionary and Dictionary.
And if we deprecated a class, it will cause warning on windows. So we rename it to __Dictionary, and typedef in CCDeprecated.h to deprecated CCDictionary.