p2pool: export data to an external web-server

Dumps data to JSON files which can be later served by a web-server.

src/common.h

@@ -254,8 +254,9 @@ struct MinerData
 
 struct ChainMain
 {
-	FORCEINLINE ChainMain() : height(0), timestamp(0), reward(0), id() {}
+	FORCEINLINE ChainMain() : difficulty(0), height(0), timestamp(0), reward(0), id() {}
 
+	uint64_t difficulty;
 	uint64_t height;
 	uint64_t timestamp;
 	uint64_t reward;