ENYO-1369: set CORS HTTP headers & allow custom user-set HTTP headers

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>
ENYO-1369: review comments, improve doc

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>
ENYO-1369: shorten lines to ease code review

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>
Merge pull request #671 from enyojs/ENYO-1369

ENYO-1369: set CORS HTTP headers & allow custom user-set HTTP headers