fixed E_NOTICE: Undefined index: debug_mode in Logger class

Hello,
here is the fix for `E_NOTICE: Undefined index: debug_mode` issue. When somehow Storage config is empty this produces a lot of `Undefined index: debug_mode` errors in server logs which is annoying when you for example run some debug tool for catching errors in your software.

It should check first if `debug_mode` is set in `Hybrid_Auth::$config` array. If it doesn't exist it should ignore displaying this notice message as `Hybrid_Logger::error()` method is being invoked with proper message already saying: `Config storage not found when trying to init Hyrid_Auth`. Which means it should not read `Hybrid_Auth::$config["debug_mode"]` at all in this case.