Added keystrokes and menu items to change playback volume.

This will resolve #138.

Screenshot of the added menu entries. I'm not confident about what the best menu order is, so I just put the entries at the end.
![screen shot 2013-08-06 at 20 07 08](https://f.cloud.github.com/assets/3893828/921550/d8a569d2-fefd-11e2-8f07-1b1172d1f7c4.png)