Mod profiling support

Config settings:
profiling = true/false (gather statistics)
detailed_profiling = true/false (break mod times to callbacks)

Chat commands:
save_mod_profile saves current statistics in debug.txt and shows on console (on default loglevel)

src/script/lua_api/l_env.h

@@ -156,6 +156,9 @@ private:
 	// stops forceloading a position
 	static int l_forceload_free_block(lua_State *L);
 	
+	// get us precision time
+	static int l_get_us_time(lua_State *L);
+
 public:
 	static void Initialize(lua_State *L, int top);
 };