play_freq polishing

I added some documentation on the waveout structs and also added some initialization methods with sensible defaults.  An additional example was added for play_freq which shows how to make threads for simultaneous playback.  I hope you don't mind, but I also set the Sound#beep example to play for only 2 seconds instead of 10.  play_freq has an additional parameter for whether or not immediate playback is wanted.  Hopefully this all looks good, let me know if you want anything different!

- Dominic