Default Bitrate

I made a few minor changes regarding bitrate. First, CBR mode is now the default instead of VBR. The default CBR bitrate is chosen based on the number of full-bandwidth channels.
1: 96 kbps
2: 192 kbps
3: 256 kbps
4: 384 kbps
5: 448 kbps

I modified the Aften commandline to accept bitrate in kbps instead of bps (i.e. '-b 192' instead of '-b 192000').

Now you can use the '-b' option along with the '-q' option. This will use VBR mode and use the given bitrate as a maximum bitrate.