Various bug fixes for outputting a properly-formatted modified box graph

IsoChunkOffsetBox.write_fields_to_stream() was using an (unused) entry_count
 memvar. Removed the memvar and fixed the chunk_offset_array output loop.
 Also fixed up references to chunk_offset_array with "this." prefix (per
 coding conventions).

Fixed a bug in IsoEditListBox.update_box_fields(). It was calculating
 the length as 16 bytes per entry (for version 0). But it's supposed to be
 12 bytes per entry (2 32-bit vals and 2 16-bit vals).

The IsoFullBox.to_string() is now appending the version and flags values
 to the output string.

Tested with B-AVC_MP4_MP_SD-03.MP4 and ran the output file through an
 mp4 parser to ensure proper parsing.
Ran binary diff with the input stream and verified a number of fields
 were modified as expected.