MINOR: lua: create and register HTTP class

This class is accessible via the TXN object. It is created only if
the attached proxy have HTTP mode. It contain all the HTTP
manipulation functions:

 - req_get_headers
 - req_del_header
 - req_rep_header
 - req_rep_value
 - req_add_header
 - req_set_header
 - req_set_method
 - req_set_path
 - req_set_query
 - req_set_uri

 - res_get_headers
 - res_del_header
 - res_rep_header
 - res_rep_value
 - res_add_header
 - res_set_header

include/types/hlua.h

@@ -15,6 +15,7 @@
 #define CLASS_CONVERTERS "Converters"
 #define CLASS_SOCKET   "Socket"
 #define CLASS_CHANNEL  "Channel"
+#define CLASS_HTTP     "HTTP"
 
 struct session;