ENH: allow choosing of audioDriver (as well as audioLib) from prefs

audioLib is the python lib that will be used. Some libs may also support
a choice of underlying sound driver (e.g. ASIO, or DirectSound...)
Merge branch 'master' of github.com:psychopy/psychopy
ENH: to allow psychopy.prefs to be changed programmatically

Should no longer use the Preferences() class but the psychopy.prefs instance::

        import psychopy
        psychopy.prefs.general['audioLib'] = ['pyo','pygame']
        print(prefs) #prints vals and file info