Added per-track duration logic for fragmented mp4s and supporting logic

IsoFileContainerBox now has get_track_duration/get_track_duration_seconds ()

All get_*_duration methods in IsoFileContainerBox take a flag to consider
 edit list boxes in the duration calculation.

Added get_track_fragment(track_id) to IsoMovieFragmentBox.

Tested with B-AVC_MP4_MP_SD-05.MP4 and checked the duration calculation
 logic. It's reporting 180.06 seconds for each track, which is correct.