+ use GNU Radio preferences file to set default input and output audio device, if provided; + use gr::logger for all non-debug messages; + case-insensitive string find with desired audio device name; + fixes buffer allocation bug with low sample rates; + allows using a specific (named) audio device, or the default; + handles the case when the selected audio device becomes unavailable (e.g., a USB stick is removed while in use); + if no audio device name is provided, uses the default audio device as found in System Preferences::Sound; + handles the case when the default audio device is in use, and the user changes that audio device in System Preferences::Sound, by internally resetting to use the newly selected audio device; + all non-Apple names are now lower_case, not CamelCase; + move osx_impl functions to gr::audio::osx, and use them correctly; + install osx_impl.h to expose gr::audio::osx functions, but iff OSX audio is enabled.