doc: rename iojs-* groups to nodejs-*
As per #2525 (closed) a bunch of WGs are renamed from iojs-* to nodejs-*. Update the WORKING_GROUPS.md to match.
Note specifically iojs-cn and iojs-tw were renamed to nodejs-zh-CN and nodejs-zh-TW respectively.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
As per #2525 (closed) a bunch of WGs are renamed from iojs-* to nodejs-*. Update the WORKING_GROUPS.md to match.
Note specifically iojs-cn and iojs-tw were renamed to nodejs-zh-CN and nodejs-zh-TW respectively.