Added timerange-to-sample/offset logic: IsoSampleTableBox.get_access_points_for_range()

AccessPoint now uses movie time units instead of floats and now has a sample
 field.

Added IsoSyncSampleBox.sync_sample_for_sample() to return the sync point for a
 given sample (to support get_access_points_for_range())

Tested with non-segmented dlna-provided content and compared with the
 corresponding index file. Time/byte range matches expected values.