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/src/map.c b/src/map.c
index 3fb84ac..eea3f4f 100644
--- a/src/map.c
+++ b/src/map.c
@@ -114,8 +114,8 @@
  * This function choose the indexation type (ebtree or list) according with
  * the type of match needed.
  */
-static int sample_load_map(struct arg *arg, struct sample_conv *conv,
-                           const char *file, int line, char **err)
+int sample_load_map(struct arg *arg, struct sample_conv *conv,
+                    const char *file, int line, char **err)
 {
 	struct map_descriptor *desc;