Add command-line control of the logging level.
Add GUID to the output of the 'select' command.
Ensure the log_callback is registered before logging a message. Default log implementation doesn't print anything.
Destructor should be virtual.
No need to 'extern "C"' the default_log function.
Add -DHAVE_SNPRINTF to the OSX build flags to prevent linker errors with certain versions of libpcap.
Describe some more basic steps in the README to help those unfamiliar with git and cmake.