initial prototype of formula update to enable Intl in node

When https://github.com/joyent/node/pull/7719 lands there will be simpler ways to build Intl support in node using ICU. This is a *prototype* for testing and comment. It will not work against the real HEAD until this PR lands. Opening the PR to get initial feedback. 
@mikemcquaid I can mention it, but Node's policy (from discussion in the linked PR and linked bugs) seems to be static link, bundle all deps. There wasn't interest in using dynamic linking on platforms that already have ICU+headers, so probably there wouldn't be much interest in linking against libraries that are not easily accessible API.