Changed IsoFileContainerBox.get_duration() to identify a single track for duration calculations

Testing with B-AVC_MP4_MP_SD-01.MP4 revealed an inaccuracy in the existing
 logic. Adding the longest TrackFragment duration from each MovieFragment
 results in a duration longer than any single track (due to differences in
 track samples between fragments). Changed the logic to use the first video
 track (or first audio track if no video track) and use that as the duration
 of the movie.

The returned length of B-AVC_MP4_MP_SD-01.MP4 now matches what QuickTime
 reports.

Also disabled some debug logging.