added: TimeZone struct (core)

this class has information about a time zone (utf offset, daylight saving time)
       and methods for converting between UTC and local time
       structs User and Config has a TimeZone object
       System::ToLocal() and System::ToUTC() uses it for converting
       (depending whether a user is logged or not)


git-svn-id: svn://ttmath.org/publicrep/winix/trunk@842 e52654a7-88a9-db11-a3e9-0013d4bc506e

core/session.cpp

@@ -75,7 +75,6 @@ void Session::Clear(bool clear_plugin_data)
 	last_time_get = 0;
 	start_date.Clear();
 	last_date.Clear();
-	tz_add        = 0;
 
 	log_buffer.Clear();
 	last_css.clear();