Add feature to set nominal rate to device

now it is possible to set nominal rate like this:

    # returns a default_output_device which's nominal rate is 48000.0
    CoreAudio.default_output_device(:nominal_rate => 48000.0)

    # returns a default_input_device which's nominal rate is 48000.0
    CoreAudio.default_input_device(:nominal_rate => 48000.0)

    # returns a devices which's nominal rate is 48000.0
    CoreAudio.devices(:nominal_rate => 48000.0)

if no sample_rate is given, device(s) which has default sample rate is returned,
so this commit destroy no backward compatibility.