Allow to set custom video bitrate - code included

[Matho]

Hi.

At first thanks for this library. The produced video with default settings (default bitrate) is too big, we needed to make the filesize smaller. I was unable to specify video bitrate in existing aperture code, so i did a little library rewrite based on some projects I have found on internet. It is working for us, we are able to specify audio/video bitrate now. Feel free to reuse and integrate changes in this commits. https://github.com/Matho/aperture/commits/master

Martin