MINOR: lua: wrapper for converters

This patch implements a wrapper to give access to the converters
in the Lua code. The converters are used with the transaction.
The automatically created function are prefixed by "conv_".
diff --git a/include/types/hlua.h b/include/types/hlua.h
index 77f10ac..eea9136 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -12,6 +12,7 @@
 #define CLASS_CORE     "Core"
 #define CLASS_TXN      "TXN"
 #define CLASS_FETCHES  "Fetches"
+#define CLASS_CONVERTERS "Converters"
 #define CLASS_SOCKET   "Socket"
 #define CLASS_CHANNEL  "Channel"