MINOR: lua: register and execute sample-fetches in LUA

This patch permits to write LUA samples fetches. Note that
all the fethes declared trough LUA are automatically prefixed
by "lua.".
diff --git a/include/types/hlua.h b/include/types/hlua.h
index 1241778..a4d1444 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -52,6 +52,15 @@
 	int function_ref;
 };
 
+/* This struct contains the lua data used to bind
+ * Lua function on HAProxy hook like sample-fetches
+ * or actions.
+ */
+struct hlua_function {
+	char *name;
+	int function_ref;
+};
+
 /* This struct contains the pointer provided on the most
  * of internal HAProxy calls during the processing of
  * rules, converters and sample-fetches. This struct is