HandlerFeedGenerator: Add caching

handlers/handlerfeedgenerator.h

@@ -9,7 +9,7 @@ class HandlerFeedGenerator : public Handler
 
   protected:
 	std::vector<EntryRevisionPair> fetchEntries(std::vector<std::string> categories);
-	Response generateAtom(const std::vector<EntryRevisionPair> &entries, std::string atomtitle);
+	std::string generateAtom(const std::vector<EntryRevisionPair> &entries, std::string atomtitle);
 	Response generateRss(const std::vector<EntryRevisionPair> &entries);
 
   public: