Rhiju/fix json io

[rhiju]

Fixing JSON i/o in latest MATLAB and with latest jsonlab. Resolves #21 and replaces PR #24.

• [x] Update JSONlab based on https://github.com/fangq/jsonlab
• [x] Tiny last cleanup that seems to fix JSON i/o for blank segid.
• [x] Unit test (JSON_IO_Test.m)!