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
hmm, let's leave it for now then.  Can you add nodoc to it so it doesn't show up in the generated RDoc?