Provide surround sound support

[cmarrin]

Allow files with 5.1 surround sound to be converted correctly (i.e. AC3 passthrough). From the original source code handover notes:

For QuickTime files, the Apple TV recognizes AC3 data with an "ac-3" audio atom. For MP4 files, check the work the HandBrake team has put into AC3-in-MP4. One of the last releases of VisualHub introduced dual-audio files. Meaning, one video track, and two audio tracks - one stereo AAC, one surround AC3. I accomplished this by modifying Johan's movmux tool (disabler) to properly construct a .mov file to the Apple TV's liking from a h264/ac3 .mov file and a stereo AAC .m4a file. Because of channel-order problems with ffmpeg at the time, re-encoding AC3 wasn't a viable option, so only proper AC3 audio tracks could be used and passed through.