MINOR: lua: map system integration in Lua

This patch cretes a new Map class that permits to do some lookup in
HAProxy maps. This Map class is integration in the HAProxy update
system, so we can modify the map throught the socket.
diff --git a/include/types/hlua.h b/include/types/hlua.h
index 82e80c4..357f7ab 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -16,6 +16,7 @@
 #define CLASS_SOCKET   "Socket"
 #define CLASS_CHANNEL  "Channel"
 #define CLASS_HTTP     "HTTP"
+#define CLASS_MAP      "Map"
 
 struct stream;