doc: add WG links in WORKING_GROUPS.md & fix nits

Related to https://github.com/iojs/evangelism/issues/24

PR-URL: https://github.com/iojs/io.js/pull/1113


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Mikeal Rogers <mikeal.rogers@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>