ENYO-2843: replace JSON.parse|stringify by enyo.json.parse|stringify

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>
Merge pull request #442 from enyojs/ENYO-2843

ENYO-2843: replace JSON.parse|stringify by enyo.json.parse|stringify