MINOR: lua: add class server

This class provides the access to the server struct, it allows
some manipulations and retrieve informations.
diff --git a/include/types/hlua.h b/include/types/hlua.h
index 17cf141..afe870a 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -20,6 +20,7 @@
 #define CLASS_APPLET_TCP   "AppletTCP"
 #define CLASS_APPLET_HTTP  "AppletHTTP"
 #define CLASS_PROXY        "Proxy"
+#define CLASS_SERVER       "Server"
 
 struct stream;