Log file specified via command line parameter did not work

This pull request fixes an issue concerning logging. Specifying  a log file via command line parameter did not work because open_log_file() method was never called in that case. Additionally that method did access $this->config['log_file'] and not the generic $this->log_file.

So, before logging did only work when specified via config file. Now it works in both cases.

Cheers
Etienne