Change log level at runtime

* Added toggle_log_level to CLILogging which toggles the log level between debug and its original value
* Allow use_log_level_option to accept an options hash and install a signal handler which calls toggle_log_level upon receing the given signal

This implements #30
The sleeps of one second where not necessary. 0.1s seem to be enough, so I went for the private toggle_log_level.